MOuntain building

Question 1

Mountain Building: overview?

Answer 1

Based on mode of origin, Four types of mountains

1. Folded Mountains
2. Block Mountains
3. Dome Mountains
4. Volcanic Mountains

Question 2

Dome Mountains?

Answer 2

Dome mountains form when large globs of magma float up from beneath the crust and push up surface rocks, creating a rounded swelling in the crust. Once the magma cools, it creates a large dome of harder rock under the surface, which erosion sometimes reveals. eg. Batholithic and Laccolithic domes

Question 3

Volcanic Mountains?

Answer 3

AKA Mountains of Accumulation as they are formed due to accumulation of volcanic materials

different types: cinder cones, composite cones etc. (refer f/c geomorphology)

Question 4

Block Mountains: about?

Answer 4

• Block mountains, also known as faultblock mountains, are the result of faulting caused by tensile and compressive forces motored by endogenetic forces coming from within the earth. Block mountains are also called as horst mountains
• Block mountains represent the upstanding parts of the ground between two faults or on either side of a rift valley or a graben. Essentially, block mountains are formed due to faulting in the ground surface.
• Block mountains are generally of two basic types e.g.

(i) tilted block mountains having one steep side represented by fault scarp and one gentle side and
(ii) lifted block mountains represent real horst and are characterized by flattened summit of tabular shape and very steep side slopes represented by two boundary fault scarps.
* Block mountains are found in all the continents e.g.
(i) Sierra Navada mountain of California (USA) is considered to be the most extensive block mountain of the world. This mountain extends for a length of 640 km (400 miles) having a width of 80 km (50 miles) and the height of 2,400 to 3,660 m (8,000 to 12,000 feet).
(ii) Vosges and Black Forest mountains (https://1drv.ms/u/s!AvN_8sA-Zf0djjjkFjkX6JVDMhYP?e=vJe9gF) bordering the faulted Rhine Rift valley in Europe,
(iii) Salt Range of Pakistan (https://1drv.ms/u/s!AvN_8sA-Zf0djjd3fk7vuC-cNSou?e=D97oNz) etc.
* There is difference of opinions among the scientists regarding the origin of block moun tains. There are two theories for the origin of these mountains viz. (1) fault theory and (ii) erosion theory.

Question 5

Block Mountains: origin : fault theory?

Answer 5

Most of the geologists are of the opinion that block mountains are formed due to faulting.

• The structural patterns of Great Basin Range mountains (https://1drv.ms/u/s!AvN_8sA-Zf0djjmAKCEUQhE8hcU0?e=uy0st4) of Utah province (USA) were closely studied by Clarence King and G.K. Gilbert who named these mountains as faulted blocks (between 1870 and 1875 A.D.). Since then the mountains formed due to large-scale faulting were named block mountains.
• Later on G.D. Louderback opined that Basin Range mountains were formed due to faulting and tilting in the ground surface.
• W.M.Davis also advocated for the fault theory of the origin of block mountains.

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiDf2DbNU5FuZrLu?e=DJMKjP

Block mountains are formed in a number of ways.

(i) Block mountains are formed due to up ward movement of middle block between two normal faults (fig. 13.1). The upthrown block is also called as horst. The summital area of such block mountain is of flat surface but the side slopes are very steep.
(ii) Block mountains may be formed when the side blocks of two faults move downward whereas the middle block remains stable at its place (fig. 13.1B). It is apparent that the middle block projects above the surrounding surface because of downward movement of side blocks. Such block mountains are generally formed in high plateaus or broad domes.
(iii) Block mountains may be formed when the middle block between two normal faults moves downward. Thus, the side blocks become horsts and block mountains (fig. 13.1C). Such mountains are associated with the formation of rift valleys.

Question 6

Block Mountains: origin : Erosion Theory?

Answer 6

J.F. Spurr, on the basis of detailed study of Great Basin Range mountains of the USA, opined that these mountains were not formed due to faulting and tilting, rather they were formed due to differential erosion. According to Spurr the mountains, after their origin in Mesozoic era, were subjected to intense erosion. Consequently, differential erosion resulted into the formation of existing denuded Great Basin Range mountains.

It may be pointed out that erosion theory of the origin of block mountains is not acceptable to most of the scientists because they believe that denudation may modify mountains but cannot form a mountain. In fact, deformatory process play major role in the origin of block mountains.

Question 7

Folded Mountains: intro?

Answer 7

• Folded mountains are formed due to folding of crustal rocks by compressive forces generated by endogenetic forces.
• These are the highest and most extensive mountains of the world and are found in all the continents.
• The distributional pattern of folded mountains over the globe denotes the fact that they are generally found along the margins of the continents either in north south direction or east-west direction.
• Rockies, Andes, Alps, Himalayas, Atlas etc. are the examples of folded mountains.
• Folded mountains are classified on various bases as follows.
  
  • Folded mountains are divided into 2 broad categories on the basis of the nature of folds.
    
    • Simple folded mountains with open folds- Such mountains are characterized by well developed system of anticlines and synclines wherein folds are arranged in wave-like pattern. These mountains have open and relatively simple fols.
    • Complex folded mountains represent very complex structure of in tensely compressed folds. Such complex structure of folds is called ‘nappe’. In fact, complex folded mountains are formed due to the formation of recumbent folds caused by powerful compressive forces.
  • Folded mountains are classified on the basis of age into
    
    • young folded mountains (which are least affected by denudational processes) and
    • mature folded mountains. Mature folded mountains are characterized by monoclinal ridges and valleys.
    • It may be pointed out that it is difficult to find true young folded mountains because the process of mountain building is exceedingly slow process and thus denudational processes start denuding the mountains right from the beginning of their origin.
  • On the basis of the period of origin folded mountains are divided into
    
    • old folded mountains: All the old folded mountains were originated before Tertiary period. The folded mountains of Caledonian and Hercynian mountain building periods come under this category. These mountains have been so greatly denuded that they have now become relict folded mountains, for example, Aravallis, Vindhyachal etc.
    • new folded mountains. The Alpine folded mountains of Tertiary period are grouped under the category of new folded mountains, for example, Rockies, Andes, Alps, Himala yas etc.

Question 8

Folded Mountains: characterestics?

Answer 8

(1) Folded mountains are the youngest mountains on the earth’s surface.
(2) The lithological characteristics of folded mountains reveal that these have been formed due to folding of sedimentary rocks by strong compressive forces. The fossils, found in the rocks of folded mountains denote the fact that the sedimentary rocks of these mountains were formed due to deposition and consolidation of sediments in water bodies mainly in oceanic environment because the argillaceous rocks of folded mountains contain marine fossils.
(3) Sediments are found upto greater depths, thousands of metres (more than 12,000 metres). Based on this fact some scientists have opined that the sediments involved in the formation of sedimen Lary rocks of folded mountains might have been deposited in deep oceanic areas but the marine fossils found in the rocks belong to such marine organisms which can survive only in shallow water or shallow sea. It means that the sedimentary rocks of folded mountains were deposited in shallow seas.

The sea bottoms were subjected to continuous subsidence due to gradual sedimentation. Thus, the greater thickness of sediments could be possible due to continuous sedimentation and subsidence and consequent consolidation of sediments due to ever increasing superincumbent load.

(4) Folded mountains extend for greater lengths but their widths are far smaller than their lengths, For example, the Himalayas extend from west to east for a length of 2400 km (1500 miles) but their north south width is only 400 km (250 miles). It means that folded mountains have been formed in long narrow and shallow seas. Such water bodies have been termed geosynclines and it has been established that ‘out of geosynclines have come out the mountains’ or ‘geosynclines have been cradles of mountains. According to PG. Worcester all great folded mountains stand on the sites of former geosynclines”
(5) Folded mountains are generally round in arch shape having one side concave slope and the other side convex slope
(6) Folded mountains are found along the margins of the continents facing oceans. For example, Rockies and Andes are located along the western margins of North and South Americas respectively and face Pacific Ocean. They are located in two directions eg. north-south (e.g. Rockies and Andes) and west-east directions (e.g. Himalayas). The Alpine mountains are located along the southern margin of Europe facing Mediterranean sea. If we consider former Tethys Sea, then the Himalayas were also located along the margins of the continent

Question 9

Geosyncline: about?

Answer 9

• The geological history of the continents and ocean basins denotes the fact that in the beginning our globe was characterized by two important features viz.
  
  • rigid masses and
  • (ii) geosynclines.
• Rigid masses representing the ancient nuclei of the present continents, have remained stable for considerably longer periods of time. These rigid masses are supposed to have been surrounded by mobile zones of water characterized by extensive sedimentation. These mobile zones of water have been termed ‘geosynclines’ which have now been converted by compressive forces into folded mountain ranges.
• On an average, a geosyncline means a water depression characterized by sedimentation. It has now been accepted by majority of the geologists and geographers that all the mountains have come out of the geosynclines and the rocks of the mountains originated as sediments were deposited and later on consolidated in sinking seas, now known as geosynclines. If we consider the height and thickness of sediments of the young folded mountains of Tertiary period (e.g. Rockies, Andes, Alps, Himala yas etc.), then it appears that the geosynclines should have been very deep water bodies but the marine fossils found in the sedimentary rocks of these folded mountains belong to the category of marine organisms of shallow seas. It is, thus, obvious that the geosynclines are shallow water bodies characterized by gradual sedimentation and subsidence
• Based on above facts geosynclines can now be defined as follows: Geosynclines are long but narrow and shallow water depressions characterized by sedimenta tion and subsidence
• The following are the general characteristics of geosynclines.
  
  • Geosynclines are long, narrow and shallow depressions of water.
  • These are characterized by gradual sedimentation and subsidence.
  • The nature and patterns of geosynclines have not remained the same throughout geological history rather these have widely changed. In fact, the location, shape, dimension and extent of geosynclines have considerably changed due to earth movements and geological process.
  • Geosynclines are mobile zones of water.
  • Geosynclines are generally bordered by two rigid masses which are called forelands.

Question 10

Geosyncline: Theories and Geographers’ concept: list?

Answer 10

1. Concept of Hall and Dana
2. Concept of E. Haug
3. Concept of JW Evans
4. Schuchert’s classification
5. Concept of Arthur Holmes
6. Concept of H. Stille

Question 11

Geosyncline: Theories and Geographers’ concept: Concept of Hall and Dana?

Answer 11

studied the folded mountains and postulated that the sediments of the rocks of folded mountains were of marine origin. These rocks are deposited in long, narrow and shallow seas. Dana named such water bodies as geosynclines. He defined, for the first time. geosynclines as long, narrow and shallow and sink ing beds of seas.

Hall elaborated the concept of geosynclines as advanced by Dana. He presented ample evidences to show relationship between geosynclines and folded mountains. He opined that the rocks of folded moun tains were deposited in shallow seas. According to Hall the beds of geosynclines are subjected to sub sidence due to continuous sedimentation but the depth of water in the geosynclines remains the same

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiF-oheZvUotX2Ii?e=bFksVu

Question 12

Geosyncline: Theories and Geographers’ concept: E. Haug?

Answer 12

• ‘If the idea of geosynclines is due to Hall and Dana, the theory of their development is really due to Haug”.
• He defined geosynclines as long and deep water bodies.
• According to Haug ‘geosynclines are relatively deep water areas and they are much longer than they are wide.
• He drew the palaeogeographical maps of the world and depicted long and narrow oceanic tracts to demonstrate the facts that these water tracts were subsequently folded into mountain ranges (fig. 13.3).
• He further postulated that the positions of the present day mountains were previously occupied by oceanic tracts i.e. geosynclines. Geosynclines existed as mobile zones of water between rigid masses.
• He identified 5 major rigid masses during Mesozoic era e.g. (1) North Atlantic Mass, (ii) Sino-Siberian Mass, (iii) Africa-Brazil Mass, (iv) Australia-India-Mada gascar Mass and (v) Pacific Mass. He located 4 geosynclines between these ancient rigid masses e.g. (1) Rockies geosyncline, (ii) Ural geosyncline, (iii) Tethys geosyncline and (iv) Circum-Pacific geosyncline.
• According to Haug there is systematic sedimentation in the geosynclines. The littoral margins of the geosynclines are affected by transgressional and regressional phases of the seas. The marginal areas of the geosynclines have shallow water where in larger sediments are deposited whereas finer sediments are deposited in central parts of the geosynclines.
• The sediments are squeezed and folded into mountain ranges due to compressive forces coming from the margins of the geosynclines.
• He has further remarked that it is not always necessary that all the geosynclines may pass through the complete cycle of the processes of sedimentation, subsidence, compression and folding of sediments. Some times, no mountains are formed from the geosynclines inspite of continuous sedimentation for long duration of geological time.
• Crticism: Though the contributions of Haug in this regard are praiseworthy as he developed the concept of geosynclines but his theory suffers from certain serious drawbacks and confusing ideas about them.
  
  • His palaeogeographical map (fig. 13.3) of Mesozoic era depicted unbelievable larger extent of rigid masses (land areas) in comparison to geosynclines (oceanic areas). Questions arise, as to what happened to such extensive land masses after Mesozoic era? Where did they disappear? Haug could not explain these and many more Questions.
  • His geosynclines as very deep oceanic tracts are also not acceptable because the marine fossils found in the folded mountains belong to the group of marine organisms of shallow seas

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiLiOH-kqmbuHQ3z?e=w4jIPE

Question 13

Geosyncline: Theories and Geographers’ concept: JW Evans?

Answer 13

• According to Evans the geosynclines are so varied that it becomes difficult to present their definite form and location. The form and shape of geosynclines change with changing environmental conditions.
  
  • A geosyncline may be narrow or wide. It may be of different shapes.
  • There may be several alternative situations of geosynclines e.g. (1) it maybe between two land masses (example, Tethys geosyncline between Laurasia and Gondwanaland), (ii) it may be in front of a mountain or a plateau (for example, resultant long trench after the origin of the Himalayas, this depression was later on filled with sediments to form Indo-Gangetic Plains), (iii) it may be along the margins of the continents, (iv) it may be in front of a river mouth etc.
• The beds of geosynclines are subjected to gradual subsidence because of sedimentation. According to Evans all the geosynclines irrespective of their varying forms, shapes and locations are characterized by twin proc esses of sedimentation and subsidence.
• Geosynclines, after long period of sedimentation, are squeezed and folded into mountain ranges.

Question 14

Geosyncline: Theories and Geographers’ concept: Schuchert?

Answer 14

He attempted to classify geosynclines on the basis of their characteristics related to their size, location, evolutionary history etc. He has divided geosynclines into 3 categories.

1. Monogeosynclines
   
   • are exceptionally long and narrow but shallow water tracts as conceived by Hall and Dana.
   • The geosynclinal beds are subjected to continuous subsidence due to gradual sedimentation and resultant load.
   • Such geosynclines are situated either within a continent or along its borders.
   • These are called mono because they pass through only one cycle of sedimentation and mountain building.
   • Applachian geosyncline is considered to be the best example of monogeosynclines. In place of the Applachians (USA) there existed a long and narrow Appalachian geosyncline during pre Cambrian period. The geosyncline was bordered by highland mass known as Applachia in the east. Applachian geosynclines were folded from Ordovician to Permian periods.
2. Polygeosynclines
   
   1. were long and wide water bodies. These were definitely broader than the monogeosynclines.
   2. These geosynclines existed for relatively longer period than the monogeosynclines and these have passed through complex evolutionary histories.
   3. These are considered to have experienced more than one phase of orogenesis, consequently they may have been diversified by the production of one or more parallel geanticlines arising from their floors in the squeezing process’.
   4. They originated in positions similar to those of monogeosynclines.
   5. Rocky and Ural geosynclines are quoted as the representative examples of polygeosynclines.
3. Mesogeosynclines
   
   1. are very long, narrow and mobile ocean basins which are bordered by continents from all sides.
   2. They are characterized by great abyssal depth and long and complex geological histories.
   3. These geosynclines pass through several geosynclinal phases e g. phases of sedimentation, subsidence and folding.
   4. Mesogeosynclines are similar to the geosynclines conceived by Haug.
   5. Tethys geosyncline is the typical example of such type. Mediterranean Sea is the remnant of Tethys geosyncline. This geosyncline was folded into Alpine mountains of Europe and the Himalayas of Asia. The unfolded remaining portion of Tethys geosyncline became Mediterranean sea, an example of median mass of Kober.

Question 15

Geosyncline: Theories and Geographers’ concept: Arthur Holmes?

Answer 15

• Besides describing main characteristics of geosynclines, A. Holmes has also elaborated the causes of the origin of different types of geosynclines. He has also described the detailed processes and mechanisms of sedimentation and subsidence and consequent orogenesis.
• According to him no doubt sedimentation leads to subsidence but this process cannot account for the greater thickness of sediments in geosynclines rather earth movements can cause subsidence of high magnitude in the geosynclinal beds. He further pointed out that the process of subsidence of the geosynclinal beds was not a sudden process rather it was a gradual process. The deposition of sediments upto the thickness of 12,160 m (40,000 feet) in the Applachian geosyncline could be possible during a long period of 3,000,000,000 years from Cambrian period to early Permian period at the rate of one foot of sedimentation every 7,500 years.
• Holmes has identified 4 major types of geosynclines and has described the mode of their origin separately as given below
  
  • Formation of Geosynclines due to Migration of Magma-
    
    • According to Holmes the crust of the earth is composed of 3 shells of rocks. Just below the outer thin sedimentary layer lies (1) outer layer of granodiorite (thickness, 10 to 12 km), followed by (ii) an intermediate layer of amphibolite (thickness, 20-25 km), and (iii) a lower layer of eclogite and some peridotite.
    • He has further pointed out that migration of magmas from the intermediate layer to neighbouring areas causes collapse and subsidence of upper or outer layer and thus is formed a geosyncline.
    • It may be summarized that some geosynclines are formed due to displacement of light magmas and consequent subsidence of crustal surface.
    • Present Coral Sea, Tasman Sea, Arafura Sea, Weddell Sea and Ross Sea have been quoted as typical examples of such geosyncline. (Location of these seas: https://1drv.ms/u/s!AvN_8sA-Zf0djjo0EM8qj4heM1rW?e=kDgEJ9)
    • This concept of Holmes has been severely criticised because the transfer and displacement of magmas cannot cause subsidence to form geosynclines.
  • Formation of Geosynclines due to Metamorphism-
    
    • According to Holmes the rocks of the lower layer of the crust, as referred to above, are metamorphosed due to compression caused by converging convective currents.
    • This matamorphism increases the density of rocks, with the result the lower layer of the crust is subjected to subsidence and thus a geosyncline is formed.
    • Caribbean Sea, the western Mediterranean Sea and Banda Sea have been quoted as examples of this category of geosynclines.
    • This concept has been rejected on the ground that compression caused by convergent con vective currents would not cause metamorphism rather it would cause melting of rocks due to resultant high temperature.
  • Formation of Geosynclines due to Compression -
    
    • Some geosynclines are formed due to compression and resultant subsidence of outer layer of the crust caused by convergent convective currents,
    • Persian Gulf and Indo-Gangetic trough are considered to be typical examples of this group of geosynclines.
  • Formation of Geosynclines due to Thinning of Sialic Layer-
    
    • According to Holmes there may be two possibilities if a column of rising convective currents diverges after reaching the lower layer of the crust in opposite directions.
    • The sialic layer is stretched apart due to tensile forces exerted by diverging convective currents. This process causes thinning of sialic layer which results in the creation of a geosyncline. The former Tethys geosyncline is considered to have been formed in this manner.
    • Alternatively, the continental mass may be separated due to enormous tensile force generated by divergent convective currents. Former Ural geosyncline is supposed to have been formed due to this mechanism.

Question 16

Geosyncline: Theories and Geographers’ concept: H. Stille?

Answer 16

• Stille has classified geosynclines based on intermittent volcanic activity during their infilling phase
• H. Stille presented a significant classification of geosynclines. He divided the earth’s crust into two major divisions called Cratons and Ortho-geosyncline.
  
  • Ortho-geosynclines were depressions that separated cratons
  • Cratons are further sub-divided into hochkraton (i.e. stable continental crust) and fiefkraton (i.e. stable oceanic crust).
  • (in my words, cratons are like plates and orthogeosynclines are depressions betn them)
• Ortho-geosyncline consiste of two parallel depressions and thus are subdivided into miogeosynclines and eugeosynclines.
  
  • Eugeosynclines are characterized by intermittent volcanic activity during the process of sedimentation, whereas miogeosynclines have low volcanic activity.
• The two classes are found side by side separated by a geanticline in the middle. Miogeosynclines are now considered to be former continental margins like those fringing the Atlantic Ocean and eugeosynclines represent the inverted and deformed equivalents of ocean basins of smaller magnitude such as the marginal basins of the western part of the Pacific, the Sea of Japan, and the Sea of Okhotsk.
• So in place of Geosynclines, he suggested ‘Geoclines’ because the geosynclinal structure is not a two sided trough rather it is open towards ocean

Question 17

3 stages of origin of Geosyncline?

Answer 17

The geosynclinal history is divided into three stages viz.

1. lithogenesis : the stage of creation of geosynclines, sedimentation and subsidence of the beds of geosynclines,
   
   1. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiO9VuGsqBvCQZyV?e=79uKec
2. orogenesis: the stage of squeezing and folding of geosynclinal sediments into mountain ranges
   
   1. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiQ8Z7OznpX8rz9v?e=ismqyR
3. gliptogenesis: the stage of gradual rise of mountains, and their denudation and consequent lowering of their height

These stages would be elaborated during the discussion of geosynclinal theory of Kober

Question 18

Folded mountain building theories: list?

Answer 18

1. horizontal forces
   
   1. horizontal movement due to contraction of earth caused by cooling
      
      • Geosynclinal theory of Kober
      • Thermal contraction theory by Jeffrys
   2. horizontal moevemnt due to continental drift and movement of earth crust
      
      • contienental drift theory of Taylor and Wegner
      • Sliding continent theory of Daly
      • radioactivity theory of Joly
      • Thermal convection current theory by A. Holmes
      • Plate tectonic theory
2. vertical forces theories
   
   • eg. undulation and oscillation theory by Harmon

Question 19

Geosynclinal theory of Kober: intro

Answer 19

Famous German geologist Kober has presented a detailed and systematic description of the surface features of the earth in his book ‘Der Bau der Erde’.

His main objective was to establish relationship between ancient rigid masses or table lands and more mobile zones or geosynclines, which he called ‘orogen.’

Kober not only attempted to explain the origin of the mountains on the basis of his geosynclinal theory but he also attempted to elaborate the various aspects of mountain building e.g. formation of mountains, their geological history and evolution and development. He considered the old rigid masses as the foundation stones of the present continents.

Question 20

Geosynclinal theory of Kober: diagram?

Answer 20

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djifKp0yFkQg2o8Ao?e=N6Owbg

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djijp1g7OI1uR5sYO?e=zpqJNY

Question 21

Geosynclinal theory of Kober: assumptions?

Answer 21

1. The driving force behind compression in his theory is the force of contraction produced by cooling of earth
2. According to him present continents have grown out of rigid masses. According to Kober there were mobile zones of water in the places of present-day mountains. He called mobile zones of water as geosynclines or orogen (the place of mountain building). These mobile zones of geosynclines were surrounded by rigid masses which were termed by Kober as ‘kratogen’.
   
   1. The old rigid masses included Canadian Shield, Baltic Shield or Russian Massif. Siberian Shield, Chinese Massif, Peninsular India, African Shield, Brazilian Mass, Australian and Antarctic rigid masses.
   2. According to Kober mid-Pacific geosyncline separated north and south Pacific forelands which were later on foundered to form Pacific Ocean.
   3. Eight morphotectonic units can be identified on the basis of the description of the surface features of the earth during Mesozoic era as presented by Kober e.g. (1) Africa together with some parts of Atlantic and Indian Oceans, (ii) Indian Australian land mass, (iii) Eurasia, (iv) North Pacific continent, (v) South Pacific continent, (vi) South America and Antarctica etc.
3. He defined the process of mountain building or orogenesis as that process which links rigid masses with geosynclines. In other words, mountains are formed from the geosynclines due to the impacts of rigid masses.

Question 22

Geosynclinal theory of Kober: mechanism?

Answer 22

1. Kober has opined that mountains are formed out of geosynclines. According to Kober geosynclines, (known as orogen) are long and wide water areas characterized by sedimentation and subsidence.
2. He described the whole process of mountain building through three closely linked stages of lithogenesis, orogenesis and Gliptogenesis
3. Lithogenesis:
   
   1. preparatory stage of mt building
   2. The geosynclines are long and wide mobile zones of water which are bordered by rigid masses, which have been named by Kober as forelands or kratogen.
   3. These upstanding land masses or forelands are subjected to continuous erosion by fluvial processes and eroded materials are deposited in the geosynclines. This process of sediment deposition is called sedimentation.
   4. The everincreasing weight of sediments due to gradual sedimentation exerts enormous pressure on the beds of geosynclines, with the result the beds of geosynclines are subjected to gradual subsidence. This process is known as the process of subsidence
   5. These twin processes of sedimentation and resultant subsidence result in the deposition of enormous volume of sediments and attainment of great thick ness of sediments in the geosynclines.
4. Orogenesis:
   
   1. Both the forelands start to move towards each other be cause of horizontal movements caused by the force of contraction resulting from the cooling of the earth.
   2. The compressive forces generated by the movement of forelands together cause contraction, squeezing and ultimately folding of geosynclinal sediments to form mountain ranges.
   3. The parallel ranges formed on either side of the geosyncline have been termed by Kober as randketten (marginal ranges)
   4. If compressive forces are weak or moderate, only the marginal sediments of the geosyncline are folded to form two marginal randketten (marginal ranges) and middle portion of the geosyncline remains unaffected by folding activity (thus remains unfolded). This unfolded middle portion is called zwischengebirge (betwixt-mountains) or median mass
   5. Alternatively, if the compressive forces are acute, the whole of geosynclinal sediments are compressed, squeezed, buckled and ultimately folded and both the forelands are closeted. This process introduces complexity in the mountains because acute com pression results in the formation of recumbent folds and nappes.
5. Gliptogenesis: characterized by gradual rise of mountains and their denu dation by fluvial and other processes. Continuous denudation results in gradual lowering of the height of mountains.
6. Kober has identified 6 major periods of mountain building.
   
   1. Three mountain building periods, about which very little is known, are reported to have occurred during pre-Cambrian period.
   2. Palaeozoic era saw two major mountain building periods - the Caledonian orogenesis was completed by the end of Silurian period and the Variscan orogeny was culmi nated in Permo-Carboniferous period.
   3. The last (6th) orogenic activity known as Alpine orogeny was completed during Tertiary epoch

Question 23

Geosynclinal theory of Kober: examples from mountains around the world?

Answer 23

• According to Kober the Alpine mountain chains of Europe can well be explained on the basis of median masses.
  
  • According to him Tethys geosyncline was bordered by European land mass in the north and by African rigid mass in the south. The sediments of Tethys geosyncline were compressed and folded due to movement of European landmass (foreland) and African rigid mass (foreland) together in the form of Alpine mountain system
  • According to Kober the Alpine mountain chains were formed because of compressive forces coming from two sides (north and south).
  • Betic Cordillera, Pyrenees, Province ranges, Alps- proper, Carpathians, Balkan moun tains and Caucasus mountains were formed due to northward movement of African foreland (fig. 13.8).
  • On the other hand, Atlas mountain (north-west Africa), Apennines, Dinarides, Hellenides and Taurides were formed due to southward movement of European landmass
  • The median masses located in the Alpine mountain system: Hungarian median mass is located between two mountain ranges- Carppathians and Dinaric Alps (Dinarides)- folding in opp directions i.e. N & S
  • Mediterranean Sea is in fact an example of median mass between Pyreness Provence Ranges in the north and Atlas mountains and their eastern extension in the south. Corsica and Sardinia are remnants of this median mass.
  • Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiVw8Jq35K92_7UM?e=RcWy0a
  • Mountains on Europe map: https://1drv.ms/u/s!AvN_8sA-Zf0djju5FazJC9iRCzbD?e=zv2ZWS
  • European Mts. : https://1drv.ms/u/s!AvN_8sA-Zf0djjx_MqAWSKYUWnCT?e=LnAF1r
• Asian Mountain Range
  
  • Anatolian plateau between Pantic and Taurus ranges is another example of median mass.
  • Similarly, further east ward, Iranian plateau is a median mass between Zagros and Elburz mountains.
  • Asiatic Alpine ranges begin from Asia minor and run upto Sunda Island in the East Indies.
  • Asiatic folded mountains including the Himalaya were formed due to compression and folding of sediments of Tethys geosyncline caused by the movement of Angaraland and Gondwana Forelands together
  • ranges, which were formed by the northward compression, include Caucasus, Pantic and Taurus (of Turkey), Kunlun, Yannan and Annan ranges,
  • ranges, which were formed by the southward compression, include Zagros and Elburz of Iran. Oman ranges, Himalayas, Burmese ranges
  • Tibetan plateau is a fine example of median mass between Kunlun-Tien-Shan and the Himalayas.
  • Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djibJuYx13BmLtv7H?e=WbJ2CP
  • Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djj0qX2R8ptyI9sCg?e=ZtMYXP

Question 24

Geosynclinal theory of Kober: Median Mass?

Answer 24

If compressive forces are weak or moderate, only the marginal sediments of the geosyncline are folded to form two marginal randketten (marginal ranges) and middle portion of the geosyncline remains unaffected by folding activity (thus remains unfolded). This unfolded middle portion is called zwischengebirge (betwixt-mountains) or median mass

median mass may be in various forms e.g.

(i) in the form of plateau (examples, Tibetan plateau between Kunlun and Himalaya, Iranian plateau between Zagros and Elburz, Anatolian plateau between Pantic and Taurus, Basin Range between Wasatch ranges and Seirra Navada in the USA):
(ii) in the form of plain (example, Hungarian plain between Carpathians and Dinaric Alps), and
(iii) in the form of seas (examples. Mediterranean Sea be tween African Atlas mountains and European Al pine mountains, Caribbean Sea between the moun tain ranges of middle America and West Indies).

Question 25

Geosynclinal theory of Kober:evaluation?

Answer 25

Though Kober’s geosynclinal theory satisfac torily explains a few aspects of mountain building but the theory suffers from certain weaknesses

• The force of contraction, as envisaged by Kober, is not sufficient to cause mountain building In fact, very extensive and gigantic mts. like the Alps, the Himalayas, the Rockies and the Andes cannot be formed by the force of contraction gener ated by cooling of the earth
• Kober’s theory some how explains the west-east extending mountains but north-south extending mountains (Rockies and Andes) cannot be explained on the basis of this theory

Inspite of a few inherent limitations and weaknesses Kober is given credit for advancing the idea of the formation of mountains from geosynclinal sediments

Question 26

Thermal Contraction Theory of Jeffreys: intro?

Answer 26

Jeffreys, a strong exponent of contraction theory, postulated his ‘thermal contraction theory’ to explain the origin and evolution of major reliefs of the earth’s surface (continents, ocean basins, mountains, island arcs and festoons) but his major objective was to explain the origin and distributional patterns of mountain systems of the globe.

Question 27

Thermal Contraction Theory of Jeffreys: axioms?

Answer 27

1. The forces responsible for mountain building process were
   
   1. forces coming through the cooling of earth, which resulted in contraction
   2. force created due to slowing down of earth’s rotation, which resulted in contraction of earth’s diameter
2. earth is composed of several concentric shells.
3. The cooling and resultant contraction takes place layer after layer but the cooling is effective upto the depth of only 700 km from the earth’s surface.

Question 28

Thermal Contraction Theory of Jeffreys: mechanism?

Answer 28

1. Within the zone of 700 km from the earth’s surface every upper layer has cooled earlier and more than the layer immediately below the upper layer. As a result it contracted more than the lower layer as well. After maximum cooling and result­ant contraction of the upper layer lower layer just lying below the upper layer begins to cool and contract, with the result already cooled and contracted upper layer becomes too large to fit in with the still cooling and contracting lower layer.
2. The layer lying over the level of no strain is too big to fit with the lower layer and hence the upper layer has to collapse on the lower layer so that it can fit with the lower layer. This process (collapse of upper layer on lower layer) results in the decrease in the radius of the earth which causes horizontal compressive stress which leads to buckling and folding of the rocks of upper layer. Thus, the mountains are formed. Further the lower layer stretches which causes fractures and fissures resulting into break­ing of rocks. This mechanism allows further collapse of the already cooled outer layer and thus already formed mountains are subjected to further rise in height.
3. Period of Mountain Building
   
   1. the process of aforesaid mechanism of mountain build­ing is not always active throughout the geological periods rather is confined to certain periods only.
   2. There is continuous accumulation of compressive and tensile forces resulting from contraction of the earth due to cooling and this process continues until the accumulated forces exceed the rock strength.
   3. When this state (when accumulated compressive and tensile forces exceed the rock strength) is reached, folding and faulting are introduced and the process of mountain building sets in and this process continues till the compressive and tensile forces are strong and active. When these forces become weak, mountain building stops and the period of quiescence sets in.
4. zones of Mountain building: According to Jeffreys mountain building depends upon the nature and strength of rocks. The areas having soft and elastic rocks are most affected by the process of mountain building as the rocks are easily folded by compressive forces caused by contraction but the regions having hard and less elastic rocks are affected by tensile forces and thus several faults and fractures are formed because such rocks are easily broken into blocks. It is, thus, apparent that mountain building is localized in certain zones of the globe.
5. Direction of Force:
   
   1. The cooling process was more active below the oceanic crust than the continental crust because of dissimilar structure of these two zones.
   2. Thus, the rocks below the oceanic crust experienced more cooling and contrac­tion than the rocks below the continental crust. Thus, the force of contraction is directed from oceanic crust towards the continental crust. This mechanism results in the formation of mountains along the continental margins parallel to the oceans. Rockies and Andes are the examples of such situation.
   3. the compressive force generated by contraction of the earth due to cooling was directed from oceanic areas towards the continental areas almost at right angle and thus the mountain ranges were formed parallel to the oceanic areas.

Question 29

Thermal Contraction Theory of Jeffreys: Evaluation?

Answer 29

1. The force of contraction resulting from the cooling of the earth is not sufficient enough to account for the origin and evolution of major surface reliefs of the globe. A. Holmes has remarked that ‘the calculated reduction of area (by Jeffreys) is seriously in deficit of the amount to explain mountain building.’
2. The concept of cooling of the earth in the system of concentric shells (layers) is erroneous and is not acceptable.
3. The impact of decrease in the speed of rotation of the earth on mountain building is doubtful.
4. As per thermal contraction theory of Jeffreys the continents and oceans should have been uniformly distributed as the earth was contracted from all sides but presently there is uneven distribution of continents and oceans.
5. According to this theory the situation of mountains should always be parallel to the oceans. The arrangement of the Rockies and Andes is justified on the basis of this theory but the arrangement of Euro­pean Alpine mountains and the Himalayas cannot be explained.
6. According to this theory there should not be any definitive distributional pattern of mountains as they may be formed everywhere because all parts of earth’s crust experienced contraction but contrary to this mountains are found in certain patterns e.g., along the margins of the consents extending either north- southward or west-eastward.

Question 30

Continental slide theory of Daly: intro?

Answer 30

Daly postulated his theory of sliding conti nents in his book ‘Our Mobile Earth’ in the year 1926 to explain the origin and evolution of different relief features of the earth’s surface.

He attempted to explain salient aspects of folded mountains e.g. origin, successive upheavals, distributional patterns and orientation and extent.

Question 31

Continental slide theory of Daly: diagram?

Answer 31

https://1drv.ms/u/s!AvN_8sA-Zf0djim2sGxtQajo3vQf?e=jTEPbS

Question 32

Continental slide theory of Daly: axioms?

Answer 32

1. According to Daly a solid crust was formed just after the origin of the earth. He named this solid crust as primitive crust.
2. In early times there existed a series of ancient rigid masses which were generally situated near the poles and around the equator. These rigid masses have been named by Daly as polar and equatorial domes. Thus, there were three belts of rigid masses e.g. (i) north polar domes, (ii) equato rial domes and (iii) south polar domes.
3. These three belts of rigid masses were separated by depressed regions which were called by Daly as midlatitude furrows and primeval Pacific Ocean. These de pressed regions were, in fact, oceanic areas (or say geosynclines) the beds of which were formed of primitive crust which was formed with the origin of the earth.
4. The crust was composed of heavier granites while the substratum was formed of lighter glassy basalt. this view of Daly is isostatically totally wrong.
5. He further assumed that the water bodies occupied about half of the globe and Tethys geosyncline (northern mid latitude furrow between north polar dome and equa torial dome) ‘was a marked feature throughout much of geological time.’
6. Land masses (polar equatorial domes - rigid masses) projected above the water bodies and the polar and equatorial domes were sloping towards mid-latitude furrows (which were in fact geosynclinal tracts) and the Pacific Ocean.

Question 33

Continental slide theory of Daly: mechanism?

Answer 33

1. The main force implied by Daly for the origin of the mountains has been the force of gravity. The whole theory of Daly is based on the nature and rate of downward slide of the continents fostered by gravitational force.
2. Daly has believed in the collapse of the primitive crust but has not elaborated the mechanism of collapse. It may be surmised that the primitive crust would have been probably bad conductor of heat and so the surface temperature would have fallen soon to that of the present time but the loss of heat from the interior into the exterior part continued and hence the interior part contracted away from the outer shell or crust. Consequently, the outer crust would have collapsed on the still contracting interior due to (i) the weight of the oceanic water, (ii) the weight of geosynclinal sediments and (iii) gravitational force of the centre of the earth. It may be pointed out that the impact of gravitational force was more under the oceanic crust than the continental domes because the former was nearer to the earth’s centre. It appears (though not described by Daly) that the mid-latitudinal furrows were formed as geosynclines due to collapse of outer crust on the contracting interior of the earth and due to the gravitational force coming from the centre of the earth.
3. The sediments derived through the erosion of polar and equatorial domes were deposited by the rivers into the mid-latitudinal furrows and the Pacific Ocean (geosynclines). Continuous sedimentation and weight of the oceanic waters exerted enormous pressure on the beds of oceans (geosynclines) with the result their beds were subjected to continuous subsidence,
4. The resultant subsidence of geosynclinal beds caused lateral pressure on the continental masses, with the result they were transformed into broad continental domes known as polar and equatorial domes. As the oceanic beds were depressed downward due to gravitational force of the earth’s centre, and weight of oceanic water and geosynclinal sediments, the size of domes continued to increase.
5. The sediments of the continental domes began to expand because of increase in the size and height of the domes and consequently sediments of the domes began to lose weight and became lighter in weight.
6. In order to compensate the loss of weight of sediments of the continental domes there began underground flowage of dense materials from below the oceanic (geosynclinal beds) beds towards the continental domes. Because of this process denser materials began to accumulate in the continental domes from below.
7. Because of the repetition of the above processes the continental domes continued to grow in size and height, probably not as rapidly in the centre as towards their peripheries’. The increase in the size of domes caused pressure on the crust under the oceanic beds (geosynclinal beds). As the size of domes continued to expand, the resultant pressure on oceanic beds also continued to increase. When the tolerance limit of the oceanic crust to withstand the ever increasing pressure was crossed, the oceanic beds began to rupture and break.
8. Thus, the support of the continental domes was removed due to rupture of the oceanic beds which introduced strong tensional movements due to which larger blocks of continental mass began to slide towards the geosynclines. The geosynclinal sediments were thus squeezed and folded due to compressive force coming from the sliding continental blocks (fig. 13.10) giving birth to folded mountains.
9. It is, thus, obvious that greater the amount of slipping of continental blocks, the more geosynclinal sediments are squeezed and more and greater folded mountains are formed.
10. Daly has fur ther pointed out that the foundered continental blocks in the substratum are melted due to high temperature and thus rise in the volume of molten continental blocks causes further rise in the mountains.

Question 34

Continental slide theory of Daly: pros?

Answer 34

• The sliding continent theory of Daly also well explains the distributional patterns of folded mountains e.g. north-south and west-east extents.
• Accord ing to Daly folded mountains are formed because of squeezing and folding of geosynclinal sediments by compressive forces caused by sliding of the continental blocks towards the geosynclines. Thus,
  
  • west east extending mountains (e.g. Alpine chains and the Himalayas) were formed due to sliding of polar and equatorial domes towards mid-latitude furrow (Tethys geosyncline)
  • north-south extending mountains (e.g. Rockies and Andes) were formed due to sliding of continental masses towards Pacific Ocean.
  • Simi larly, the island arcs and festoons parallel to the Asiatic coast were formed due to sliding of Asiatic mass towards Pacific Ocean.

Question 35

Continental slide theory of Daly: cons?

Answer 35

1. The sliding continent theory presents erroneous concepts about the structure of the interior of the earth. His concept, that the outer crust is denser than the substratum, is against the evidences of seismology because it is now proven fact that the density increases with increasing depth in the inte rior of the earth.
2. Daly’s theory is based on several guesses and surmises. Why did the earth’s crust become asymmetrical? Why the continental domes were sloping towards mid-latitude furrows (geosynclines)? How was the Pacific Ocean formed? Daly does not offer any convincing explanation to these and many more questions.
3. This theory presents erroneous views about geosynclines because these are generally considered as long, narrow and relatively shallow depressions of water but Daly’s geosynclines were in fact oceans (e.g. mid-latitude furrows and Pacific Ocean). If these are accepted as geosynclines they would have never been filled with sediments and thus no moun tains could have been formed.
4. The theory provides wrong views about the mechanism and process of gravity. The theory does not throw light on the termination of pulling effects of gravity and the beginning of the rupture of the beds of the geosynclines. Thus, there is no coherence between different events related to moun tain building as envisaged by the sliding continent theory. In fact, the theory presents some piecemeal analysis of mountain building rather than a complete or perfect perspective.

Question 36

Thermal Convection Current Theory: intro?

Answer 36

Arthur Holmes postulated his thermal con vection current theory in the year 1928-29 to explain the intricate problems of the origin of major relief features of the earth’s surface.

Holmes’ major objec tives were not confined to search the mechanism of mountain building based on sound scientific back ground but were also directed towards finding scien tific explanation for the origin of the continents and ocean basins in terms of continental drift as he was opposed to the concept of permanency of the conti nents and ocean basins as envisaged by the advo cates of thermal contraction of the earth.

Question 37

Thermal Convection Current Theory: axioms/basic assumptions?

Answer 37

1. According to Holmes the earth consists of 3 zones or layers
   
   1. upper layer of granodiorite (10 to 12 km),
   2. intermediate layer (20 to 25 km) of amphibolite and
   3. lower layer of eclogite.
2. He has further grouped these three layers into two zones e.g. (i) crust consisting of upper and middle or intermediate layers and crystalline upper part of lower layer and (ii) substratum representing molten part of the lower layer.
3. Crust and substratum are composed of sial and sima respectively.
4. Generally, sial is absent in the oceanic areas.
5. Convective Currents are generated within the earth due to heat generated by presence of radioactive elements in the rocks

Question 38

Thermal Convection Current Theory: convective current origin?

Answer 38

1. The origin of convective currents within the earth depends on the presence of radioactive elements in the rocks. The disintegration of radioactive elements generates heat which causes convective currents. According to Holmes there is maximum concentration of radioactive elements in the crust but temperature is not so high because there is gradual loss of heat through conduction and radiation from the upper surface.
2. On the other hand, though there is very low concentration of radioactive elements in the substratum but the gradual accumulation of heat, produced by the radioactive elements causes convective currents.
3. The convective currents depend on two factors (i) thickness of the crust near the equator and poles and (ii) uneven distribution of radioactive elements in the crust.
4. Ascending convective currents originate under the crust near the equator because of greater thickness of crust whereas descending convection currents are originated under the polar crust because of its shallow depth.
5. The rising convective currents originating from below the continental crust are more powerful than the convective currents originating from below the oceanic crust because of greater concentra tion of radioactive elements in the continental crust.
6. the currents originating under the equatorial crust move towards the poles and thus the crusts are carried away with the convective currents.
7. The convective currents are divided into two groups on the basis of their locational aspect
   
   1. Convective currents of rising columns: The rising convective cur­rents after reaching the lower limit of the crust diverge in opposite directions. This outward or divergent move­ment introduces tensional force due to which the crust is stretched, thinned and ultimately broken and the broken crustal blocks are moved apart. The wide open area between two drifting crustal blocks in opposite directions is filled with water and thus an ocean is formed.
      
      1. According to Holmes the equatorial crust was stretched and ruptured due to divergence of rising convective currents which carried the ruptured crustal blocks towards the north and south and Tethys Sea was formed. This phase is called ‘opening of Tethys’
   2. Convective currents of falling columns: This creates tensional pull on the crust above. It may result in formation of folded mountains in case the two columns are beneath continental and oceanic crust. It may also lead to closing up of oceans as happened in the case of tethys sea.
      
      1. two sets of convergent or downward moving (descend­ing) currents brought Laurasia and Godwnaland to­gether and thus Tethys was compressed and folded into Alpine mountains. This phase is called ‘closing of Tethys’.

Thus, divergent convective currents move the crustal blocks away in opposite directions and thus create seas and oceans while convergent convective currents bring crustal blocks together and thus form mountains.

8. Geosynclines are always located above the de­scending convective currents of falling column. This is because, Geosynclines are formed due to subsid­ence of crustal blocks, mainly under continental shelves, due to compressive force generated by convergent convec­tive currents moving laterally together under continen­tal and oceanic crusts.
9. The convective mechanism is not a steady process but a periodic one, which waxes and wanes and then begins again with a different arrangement of centre’. It means that the convec­tive currents originate at several centres which are not permanent.

10. Holmes has described a cyclic pattern of thermal convective currents which includes the origin of convective currents, formation of geosynclines, sedimentation and orogenesis and further rise in the mountains.

Question 39

Thermal Convection Current Theory: mechanism?

Answer 39

According to Holmes the cyclic pattern of convective currents and related moun­tain buildings pass through three phases or stages

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djiqVnwi0t8ZOGG3B?e=fyhPpZ

1. First Stage of Lithogenesis: First stage is of the longest duration during which convective currents are originated in the sub­stratum
   
   1. The first stage, character­ized by high velocity convective currents, is in fact the preparatory stage of mountain building which is marked by the creation of geosynclines, sedimentation and subsidence of materials
   2. The rising convective currents of two centers converge under the continental shelves and thus form geosynclines due to compression coming from the convergence of two sets of lateral currents
   3. Geosynclines are subjected to continuous sedimenta­tion and subsidence due to increasing incumbent load as well as because as the sediments are pressed downward into geosynclines, these go further down­ward and are intensely heated and metamorphose into higher density material (Amphibolites are metamorphosed into eclogites) which subsides further. Thus, the falling column of downward moving convective currents is the column of increasing density.
2. Second Stage of Orogenesis: AKA stage of orogenesis.
   
   1. Second stage is marked by phenomenal increase in the velocity of convective currents but this stage is relatively of short duration.
   2. The main cause for the phenomenal increase in the velocity of convective currents is the downward movement of cold materials in the falling column and upward movement (rise) of hot materials in the rising column of convective cur­rents. Increased pressure due to metamorphism of geomaterials in the falling column of descending cur­rents increases the velocity of downward moving con­vective currents.
   3. The high velocity convergent con­vective currents buckle geosynclinal sediments and thus initiate the process of mountain building
3. Third Stage of Gliptogenesis
   
   1. Third stage is characterized by waning phase of thermal convective currents due to incoming hot mate­rials in the falling column and upward movement (rise) of colder materials in the rising column
   2. Gradually, the rising column becomes a cold column i.e., cold materi­als are accumulated at the centre of the origin of rising (upward moving) convective currents due to which these currents cease to operate and the whole mecha­nism of convective currents comes to an end.
   3. Due to this stoppage of covective cycle, mountains rise because of three factors
      
      1. The materials of the falling column start rising because of decrease in the pressure at the top of the falling column due to the end of deposition of materials.
      2. The depressed and subsided heavier materials in the falling column of descending convective currents start rising due to decrease in the weight and pressure at the top of the falling column
      3. Eclogite, which was depressed downward, gets melted due to immense heat and thus it expands. This expansion in the volume of molten eclogite causes further rise in the mountains

Question 40

Thermal Convection Current Theory: Evaluation?

Answer 40

Pros

1. explains all three stages of mountain building
2. Griggs experiments as well as modern evidences have validated existence of convective currents

Criticism

1. At the time the theory was given, it was criticised because of lack of evidence for convective currents. But it has since been validated
2. The whole mechanism of convective cur­rents depends on the heat generated by radioactive elements in the substratum (now mantle) but critics raised doubt about the availability of required amount of heat generated by radioactive ele­ments.
3. The metamorphism of amphibolites into eclogites and resultant downward movement of rela­tively denser eclogites is also a doubtful phenomenon. Even we accept the metamorphism of amphibolites into eclogites but the resultant increase in density from 3.0 to 3.4 would not be enough to depress and sink eclogites in the falling column. If desired sinking of eclogites is not possible, there would not be proper accommodation of materials brought by the horizontal convergent convective currents into the falling col­umn.
4. According to this theory convective currents are originated at few centres only under the continental and oceanic crusts but question arises, why are not they originated at all places? If this so happens, the horizon­tal movement of these currents would not be possible. The whole of the continents would be divided into several blocks as the rising convection currents origi­nating from numerous centres would break the crusts and would give birth to volcanic eruptions of various sorts. This has now been answered by Plate tectonics - rising and falling convective currents are active at plate margins

the idea of thermal convective currents conceived by A. Holmes about 80 years ago (in 1928-29) proved its worth in 1960s when the scientists were looking forward to search such a force which can explain the movement of plates. Now, the process of mountain building can be very satisfactorily explained on the basis of convective currents though not in the way as conceived by A. Holmes in 1928-29 but on the lines of plate tectonics.

Question 41

Radioactive Theory of Joly: intro?

Answer 41

Joly postulated his theory based on radioactiv­ity of certain radioactive minerals in the year 1925 in his book, ‘Surface History of the Earth’ to account for the origin and evolution of surface features of the earth. His theory is also known as thermal cycle theory or theory of the surface of the earth.

Though the main objective of Joly’s theory was to present a detailed account of the thermal history of the earth and math­ematical explanation of the structure of the interior of the earth but he also attempted to explain the problems of mountain building and the continental drift.

Question 42

Radioactive Theory of Joly: axioms/ basic assumptions?

Answer 42

1. the crust has been assumed to have been composed of sial and substratum of basalt (sima).
2. According to Joly the rocks of the earth contain radioactive elements but their distribution is not uni­form in all zones of the earth. Radioactive elements are found in abundance in sialic zone or the continental rocks but the rocks of sima forming the oceanic crusts are less radioactive.
3. He assumed the maximum thickness of sial to be 30km., with no heat transfer betn sial and sima layer beneath

Question 43

Radioactive Theory of Joly: summary/highlights?

Answer 43

1. The driving force of the mountain building as invoked by Joly is provided by expansion and contrac­tion of the substratum of the earth resulting into transgressional and regressional phases of the seas (geosynclines).
2. The expansion and contraction of the substratum are based on the mechanism of heat gener­ated by radioactive elements of the rocks.

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djivjlK2hG6A6cs9p?e=koUyRI

Question 44

Radioactive Theory of Joly: radioactivity in the Interior of earth?

Answer 44

1. The theories of both A. Holmes and Joly are based on radioactive elements but they sought their help differently e.g., Holmes used radioactive elements to explain the origin of thermal convective currents in the substratum while Joly used them to explain the melting and re-solidification of the substratum. He also implied tidal force and friction to explain continental drift.
2. In order to explain various aspects of the mechanism of radioactive elements Joly has described first the structure of the earth.
3. According to him con­tinents are made of lighter sialic materials the density of which is 2.67 while the oceanic beds are formed of heavier materials of sima having average density of 3.0. Thus, the crust has been assumed to have been composed of sial and substratum of basalt (sima).
4. According to Joly the rocks of the earth contain radioactive elements but their distribution is not uni­form in all zones of the earth. Radioactive elements are found in abundance in sialic zone or the continental rocks but the rocks of sima forming the oceanic crusts are less radioactive.
5. Continuous breakdown of certain radioactive elements like uranium, thorium etc., gener­ates heat. Though, the actual rate of heat production by radioactive elements is exceed­ingly small but it becomes sufficient enough to pro­duce appreciable result after long period of accumula­tion.
6. Under Continents
   
   1. According to Joly the disintegration of radioac­tive elements of sialic or continental rocks produces heat but it does not accumulate in the continents or sial because the total loss of heat through radiation from the sialic crust
   2. He has further pointed out that temperature increases with increasing depth. He calculated T at 30km to be 1050°C.
   3. He estimated the maximum thickness of sial to be 30km. According to him there is no transfer of heat from sima to overlying sial. Thus temperature at the outer limit of sima under the continents to be 1050°C.
7. Under Oceans
   
   1. Temperature increases with in- creasing depth in the substratum (sima) under the oceans because of accumulation of heat produced by radioactive elements. This mechanism causes tem­perature gradient at greater depth in sima
   2. The temperature becomes equal to the melting point of basalt. There is no transfer of heat from the lower part of sima to the upper part of sima so there is accumula­tion of heat in the lower layers of sima beneath the oceans.

Question 45

Radioactive Theory of Joly: Period of Transgressional Sea?

Answer 45

when the substratum reaches the molten condition (refer slide # 47), following occur

1. The expansion of sima due to melting causes increase in the radius of the globe
2. Continen­tal masses or sialic masses are raised relative to the centre of the globe.
3. sialic masses begin to sink in molten sima.
4. The level of oceanic water rises due to sinking of sialic or continental masses into liquid sima. This mechanism causes extension of oceanic water over the continental margins. This process of expansion of oceanic waters and their encroachment on-continental margins is called transgression of sea and the concerned stage is known as the phase of transgressional sea.
5. Transgression of sea results in sedimentation on the submerged continental margins. Thus, this theory of radioactivity accounts for the origin of geosynclines due to submergence of conti­nental margins during transgressional phase of sea.
6. Under the oceans, The increase in the radius and the circumference of the globe due to melting of sima produces tension in the oceanic beds which causes cracks and faults. Molten materials or molten basalts come upward through these cracks and faults. These molten basalts are then solidified and thus oceanic- islands are formed. The radioactivity theory, thus, explains the islands of the Pacific and other oceans.
7. Continental masses easily float over molten sima, consequently they are more affected by tidal force which causes westward movement of the continents. It is in this way that the radioactivity theory also de­scribes the process of continental drift.
8. Continental drift changes the position of the continents and the oceans as the former occupy the positions of the latter. This process allows the escape of heat and thus the transgressional phase comes to an end.

Question 46

Radioactive Theory of Joly: Period of Regressional Sea?

Answer 46

1. The temperature of the substratum decreases because of loss of heat due to continental drift. Thus, the cooling of the substratum results in the resolidification of molten substratum. The cooling of the substratum begins from its upper layer and continues downward and ultimately the whole of the substratum becomes solid on cooling.
2. The density of the substratum (sima), which was relatively decreased during its molten stage, again increases to regain its previous value.
3. The radius and the circumference of the globe, which were increased due to melting of the substratum, arc again shortened to their previous position, with the result the continents, which were raised relative to the centre of the globe, are again brought to their previous positions.
4. Relative increase in the density of the substratum due to resolidification causes contraction of the oce­anic bed which results in the withdrawal of oceanic waters from the continental margins. This is called the phase of regressional sea.
5. Because of the withdrawal of oceanic water previously submerged continental mar­gins (during the phases of transgressional sea) rise upward and the deposited sediments are exposed above the water level.

It may be remembered that the oceanic beds were subjected to maximum expansion during the period of transgressional phase due to melt­ing of the substratum. Similarly, the oceanic beds are also subjected to maximum contraction during the period of regressional sea due to resolidification of molten substratum.
6. Thus, contracting beds of two oceans exert lateral compression on the sediments deposited on the continental margins (geosynclines), consequently the sediments deposited during the pe­riod of transgressional sea are squeezed, buckled and folded and thus mountains are formed.

Question 47

Radioactive Theory of Joly: mechanism?

Answer 47

Temperature at top of sima layer is 1050^o C while its melting point is 1150^o C. Joly calculated that the required amount of heat to liquefy the substratum (to increase T by 100^o C as well as latent heat of fusion) would be available in 33,000,000 to 56,000,000 years. If such conditions become possible i.e. if the substra­tum reaches the molten condition, several changes take place in the earth’s structure during, what he called, Period of Transgressional Sea.** It was followed by **Period of Regressional Sea.

Joly has described two parallel processes of mountain building:

(i) The sediments deposited in the shallow seas of the continental margins are squeezed and folded due to lateral compression caused by two contracting oceanic beds,
(ii) Vertical force is pro­duced during the process of resolidification of the substratum. This vertical force raises the whole moun­tains system formed during the first process.

It is obvious that according to this theory mountains are always formed along the margins of the continents facing oceans.

The intensity of lateral pressure and consequent magnitude of folding depend on the amount of contraction of oceanic beds. It may be argued that large oceans would produce more powerful lateral compression and hence greatest mountain would face largest ocean. eg. Rockies and Andes facing Pacific Ocean

Joly also explains the period of quiescence between two periods of mountain building. The total period of two solid phases of the substratum (solid phase, molten phase and resoldification phase of the substratum) is called a revolution wherein the melting of substratum (sima) takes total time period of 33,000,000 to 56,000,000 years. It may be, thus, inferred that the process of mountain building occurs in cyclic manner wherein the period of mountain building is alternated by the period of quiescence.

Question 48

Radioactive Theory of Joly: evaluation?

Answer 48

Pros:

1. Joly’s views on the earth’s surface history are based on such reasonable premises, and are so simple in their conception, that they have met with a great deal of favour

Criticism

1. the force of expansion and contraction of the substratum (sima) due to melting and cooling respectively based on radioactive ele­ments is doubtful and perhaps is not enough to form mountains.
2. Jeffreys has demonstrated that sima (substratum) if melted at all cannot resolidify if we accept the capacity of radioactive agencies to liquefy sima.
3. Jeffreys did not agree with the 30km thickness of the continental masses as envisaged by Joly. According to Jeffreys the thickness of the conti­nental crust may not be more than 16km. If the thickness of the continental crust is accepted to be 16 km then the whole mechanism of Joly’s theory would come to a grinding halt as required amount of heat of 1150 C would not be possible at the depth of 16km.
4. Joly’s concept of cyclic nature of moun­tain building has been disputed by some critics. The theory envisages uniform periods of quiescence be­tween two periods of mountain building but this con­cept has also been disputed.
5. This theory envisages two facts about mountain building, (i) ‘The greatest mountains must face the greatest oceanic beds’, (ii) Both the margins of the continent must have mountains of the same period and both the margins should be regular. The first is validated to some extent but the second fact is not validated.
6. This theory presents erroneous concept about geosynclines. As per this theory geosynclines are al­ways formed due to submergence of continental mar­gins due to transgression of seas. It means that geosynclines should always be located around the continents. Further, Joly’s geosynclines receive sediments but do not undergo the process of subsidence. Without subsidence the enor­mous thickness of sediments of the present Alpine mountains cannot be explained.

Question 49

Continental Drift Theory: Mountain Building?

Answer 49

The frontal edges of westward drifting continental blocks of North and South Americas were crumpled and folded against the resistance of the rocks of the sea-floor (sima) and thus the western cordilleras of the two Americas (e.g. Rockies and Andes and other mountain chains associated with them) were formed.

Similarly, the Alpine ranges of Eurasia were folded due to equatorward movement of Eruasia and Africa to gether with Pennisular India (equator was passing thorough Tethys sea at that time).

(can refer f/c Geomorphology)

Question 50

Plate Tectonic Theory of Mountain Building: intro?

Answer 50

cover it frm Geomorphology f/c

Question 51

Plate Tectonics: Mountain Building: different process?

Answer 51

Plate Collision and subduction. Can be of three types depending on the plates i.e.

1. O-O

2 O-C

3 C-C

Question 52

Plate Tectonics: Mountain Building: Convergence of two Oceanic plates?

Answer 52

collision of two oceanic plates and subduction of the boundary of the plate of relatively denser materials results in the formation of the volcanic mountain arcs or island arcs and festoons, for example, island arcs and festoons formed by Japanese islands, Phillippines etc. around the western margin of the Pacific Ocean off the east coast of Asia.

The fold mountain ranges of island arcs and festoons ‘form where a section of the ocean floor is subducted in the ocean basin away from a continent i.e. where ocean floor crust is on either side of the convergent plate boundary’ (M.J. Bradshaw et al. 1978).

The best example of the formation of mountains due to collision of two oceanic plates is the situation of Japanese island arc. Mountains of Japan range in height from 3000 m to 4000 m AMSL. It may be pointed out that all the mountains of Japan are of volcanic origin. Though Japanese mountains exhibit a number of characteristic features of folded mountains but they can no longer by regarded as fold mountains like the Alps and the Himalayas.

Honshu Island represents the most characteristic example of the situation of the convergence of two oceanic plates. Honshu is bordered by Japan Trench in the east and Japan Sea in the west. The western part of the island is more frequented by volcanic activities than the eastern part. The island is characterized by two belts of metamorphic rocks on either side. It is believed that the Japan Trench was formed due to subduction of Pacific Oceanic plate under the oceanic crust to the east of Japan. According to plate tectonic theory the subducted portion of plate after reaching a depth of 100 km or more starts melting due to high temperature prevailing in the upper mantle. The magma, thus formed, ascends and appears as volcanic eruption about 200 km away from the oceanic trench. Since Japan is very close to the Japan Trench and hence western part of Japan is more frequented by volcanic activities. This process is still continuing as the Pacific plate is being continuously subducted under the oceanic crust along the Japan Trench. The eruptions of volcano in the month of June, 1991 in Japan after a dormant period of about 200 years and the eruption of Mt.Pinatubo on June 9, 1991 in Manila, Phillippines. validate the authenticity of this theory of plate tectonics. The volcanic eruptions caused by subduction of oceanic plates under the oceanic crust off the Japanese coast resulted into continuous accumulation of volcanic rocks and consequent increase in the height of island are and thus the formation of volcanic mountains could be possible.

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djixaajUNKifgy-XE?e=MWgu6N

Question 53

Plate Tectonics: Mountain Building: Convergence of Continental and Oceanic PLates?

Answer 53

The collision of continental and oceanic convergent plates results in the formation of cordillera type of folded mountains e.g. the western cordillera of North America (including the Rockies).

When one continental and the other oceanic plates collide due to their convergence along subduction or Benioff zone, the oceanic plate boundary being heavier due to comparatively denser materials is subducted below the continental plate boundary.

The sediments deposited on the continental margins are squeezed and folded due to compressive forces caused by the subduction of oceanic plate (see Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWIp-GbUTUG8Dtgw?e=UQUexb).

The Rockies and the Andes mountains were formed due to subduction of the Pacific ocean plate under the American continental plate

Question 54

Plate Tectonics: Mountain Building: Convergence of two Continental plates?

Answer 54

When two convergent plates composed of continental crusts collide against each other, the continental plate having relatively denser materials is subducted under the other continental plate having comparatively lighter materials than the former.

The resultant lateral compression squeezes and folds the sediments deposited on either side of the continental plate margins and the sediments of the geosynclines lying between two convergent continental plates and thus forms gigantic folded mountains e.g. the Alps and the Himalayas. Mountain chains were formed due to continued collision of continental plates and consequent orogenesis along several subduction zones for long periods of time.

About 70-65 million years ago (Mesozoic era) there was an extensive geosyncline, known as Tethys geosyncline, in the place of the Himalayas. Tethys geosyncline was bordered by Asiatic plate in the north and Indian plate in the south. Tethys geosyncline began to contract in size due to movement of Indian and Asiatic plates together. About 60-30 million years ago the Indian plate came very close to Asiatic plate. The Indian plate began to actively subduct under the Asiatic plate. The convergence and collision of Asiatic and Indian plates and consequent subduction of Indian plate under the former caused lateral compression due to which the sediments of Tethys geosyncline were squeezed and folded into three parallel chains of the Himalayas about 30-20 million years ago. It has been estimated that the crust has been shortened by 500 km between Asiatic and Indian plates due to convergence of two plates and subduction of Indian plate

Similarly, Alpine mountains of Europe were formed due to convergence and collision of European and Afri can plates. Since the collision of these two continen tal plates was very complex and hence the structure of the European Alpine mountains is also very complex.

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0dji1Hztbpj2bUQE2O?e=q2AbcZ

Question 55

Plate Tectonics: Mountain Building: other minor processes at work?

Answer 55

1. Mountains by Subduction:
   (i) Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zone, and, therefore, the rocks in this region are warm and weak.
   (ii) Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward.
   (iii) eg. Andes Mountain
2. Mountains by Terrane Accretion
   (i) As the ocean contracts by subduction, elevated regions within the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin.
   (ii) eg. addition of these accreted terranes has added an average of 600 km (400 miles) in width along the western margin of the North American continent, and the collisions have resulted in important pulses of mountain building.
   (iii) During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends. Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted. Where this occurs, rare slices of ocean crust, known as ophiolites, are preserved on land. A classic example is the Coast Range ophiolite of California
3. Crustal thickening during Continental Collision
   (i) neither continent being subducted to any appreciable extent. A complex sequence of events ensues that compels one continent to override the other. These processes result in crustal thickening and intense deformation that forces the crust skyward to form huge mountains with crustal roots that extend as deep as 80 km.
   (ii) As continental collisions are usually preceded by a long history of subduction and terrane accretion, many mountain belts record all three processes. Over the past 70 million years the subduction of the Neo-Tethys Sea, a wedge-shaped body of water that was located between Gondwana and Laurasia, led to the accretion of terranes along the margins of Laurasia, followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia. These collisions culminated in the formation of the Alps and the Himalayas.

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djVqRpL3VM_tQ_lRB?e=faxael

Many mountain belts were developed by a combination of these processes. For example, the Cordilleran mountain belt of North America—which includes the Rocky Mountains as well as the Cascades, the Sierra Nevada, and other mountain ranges near the Pacific coast—developed by a combination of subduction and terrane accretion.

Question 56

Plate Tectonics: Mountain Building: periods of mountain building?

Answer 56

4 major periods of mountain building have been identified

(i) pre Cambrian orogeny:

N. America: Laurentian Mts, Algoman Mts, Kilarnean Mts.

Europe: mountains of Feno-Scandia, North-West Highlands and Anglesey etc.

(ii) Caledonian orogeny: mts. formed during Silurian and Devonian periods

Europe: Taconic mountains of the Applachian system, moun tains of Scottland, Ireland and Scandinavia

Brazilides of South America,

Aravallis, Mahadeo, Satpura etc. of India.

(iii) Hercynian orogeny : mountains formed during Permian and Permocarboniferous periods,

Europe: mountains of Iberian peninsula, Ireland, Spanish Messeta, Brittany of France, South Wales, Cornwall, Mendips, Paris basin, Belgian coalfields, Rhine Mass, Bohemian plateau, Vosges and Black Forest, plateau region of central France, Thuringenwald, Frankenwald, Hartz mountain, Donbas coalfield

Asia: Variscan moun tains of Asia include Altai, Sayan, Baikal Arcs, Tien Shan, Khingan, mountains of Dzungarian basin, Tarim basin, Nanshan, Alai and Trans Alai moun tains of Amur basin, Mongolia and Gobi etc;

Australian Variscan mountains include the scattered hills in the Eastern Cordillera, New England of New Southerwales;

North American Variscan mountains include Applachians;

South American Variscan mountains are Austrian and Saalian folds of San Juan and Mendoza, mountains of Puna arc of Atacama, Gondwanides of Argentina etc.

(iv) Tertiary orogeny.: mountains formed during Teritary period,

NA: Rockies

SA: Andes

Europe: Alps, Carpathians, Pyrenees, Balkans, Caucasus, Cantabrians, Apenians, Dinaric Alps etc.

Africa: Atlas Mts

Asia: Himalayas and mountains coming out of Pamir knot- Taurus, Pauntic, Zagros, Elbruz, Kunlum etc.

Question 57

Plate Tectonics: Mountain Building: Evaluation?

Answer 57

• Theory validated oth evidences of paleomagnetism and sea floor spreading as well as satellite data.
• Also satisfactorily explains cyclic pattern of mt. building

Criticism

• can refer f/c Geomorphology- evaluation of PT

Question 58

Fundamental Concepts of Geomorphology: all the concepts?

Answer 58

1. The same physical processes and laws that operate today, operated throughout geological time, although not necessarily always with the same intensity as now – WD Thornbury
2. Geologic structure is a dominant control factor in evolution of landforms and is reflected in them. – WD Thornbury
3. Geomorphic processes leave their distinctive imprints upon landforms and each geomorphic process develops its own characteristic assemblage of landforms. –WD Thornbury
4. As the Different erosional agencies act on earth’s surface there is produced a sequence of landforms having distinctive characteristics at the successive stages of their development. –WD Thornbury
5. Geomorphic scale is a significant parameter in the interpretation of landform development and landform characteristics of geomorphic systems. OR Landscape is a function of time and space.
6. A simple Geomorphological equation may be envisaged as a vehicle for the explanation of landforms as follows: F=f(PM) dt
7. Complexity of geomorphic evolution is more common than simplicity.
8. Little of earth’s topography is older than tertiary and most of it is no older than Pleistocene.
9. Each climatic type produces its own characteristic assemblage of landforms.

Question 59

Fundamental Concepts of Geomorphology: Concept 1: statement? Headings?

Answer 59

The same physical processes and laws that operate today, operated throughout geological time, although not necessarily always with the same intensity as now

1. intro
2. Cyclic nature of earth’s history
3. active always though not always with same intensity

Question 60

Fundamental Concepts of Geomorphology: Concept 1: intro?

Answer 60

• AKA principle of Uniformitarianism, which was first formulated by renowned Scttish Geologist James Hutton in 1785.
• “Present is key to the past”
• “No vestige of a beginning and no prospect of an end”

Question 61

Fundamental Concepts of Geomorphology: Concept 1: Cyclic Nature of Earth’s History?

Answer 61

• Hutton was the first scientist who postulated the concept of cyclic nature of earth’s history. All major geological activities are repeated in cyclic manner.
  
  • For example, there have been four major periods of mountain building viz precambrian, caledonian, hercynian and tertiary periods of mountain building and each mountain building period was succeeded by a period of quiescence.
  • Similarly, glacial periods during Pleistocene ice age were separated by interglacial periods.
• There are ample evidences to validate the observations that each geological process has completed several cycles during geological past but it becomes difficult to find out as to when a particular geological process began to work and it is equally a difficult task to predict as to when a particular process would cease to work. Based on this connotation Hutton postulated his concept, ‘no vestige of a beginning: no prospect of an end.
• The examples of denudation chronology of the Applachians and Peninsular India may demonstrate the cyclic nature of earth’s history as envisaged by Hutton.
  
  • The Applachian revolution during Permian period resulted in the 1st upliftment of the Applachians which was followed by long period of active denudation culminating into the development of Schooley peneplain which was again uplifted and then was peneplained to form Shenondoah peneplain. The third phase of upliftment was again followed by active denudation resulting in the formation of Harrisberg peneplain which was again uplifted in the recent past and fourth cycle of erosion is in operation
  • Peninsular India has passed through various phases of cyclic development eg. Dharwar landscape cycle, Cuddapah-Vindhyan landscape cycle, Cambrian landscape cycle, Gondwana land scape cycle, Cenozoic landscape cycle etc.
• The processes (mainly endogenetic) which affect the earth’s crust act in a cyclic manner. Hutton believed in orderliness of nature i.e. the nature evolves in orderly course. According to him the nature is systematic, orderly, coherent and reason able ie. destruction leads to construction while construction results into destruction.
  
  • For example, denudation of uplands (destruction) leads to sedimentation in lowlying areas giving birth to alluvial plains (construction). Continuous sedimentation leads to subsidence of ground surface.
• The nature has inbuilt self regulatory mechanism known as homeostatis mechanism which acts in such a manner that any change effected by natural factors (whether endogenetic or exogenetic) is suitably compensated by changes in other components of the natural system.

Question 62

Fundamental Concepts of Geomorphology: Concept 1: contributors?

Answer 62

• Hutton
• WM Davis’ Cycle of Erosion
  
  • inspired by Hutton’s concept of cyclic nature of earth’s history, and
  • evolutionary concepts of Charles Darwin
  • presented his ‘geographical cycle of erosion’ in 1899
  • based on basic concept of ‘sequential changes in landforms through time like the evolution of an organic life’
  • Complete: https://1drv.ms/b/s!AvN_8sA-Zf0djlIbOvj9G7GHpSfy?e=Qb0Yfl
  • Youth stage: https://1drv.ms/u/s!AvN_8sA-Zf0djk9kKZmM9ay8-SPC?e=CyZzl3
  • Mature stage: https://1drv.ms/u/s!AvN_8sA-Zf0djlAQUXdjCibY9SNH?e=m0CRZB
  • Old Stage: https://1drv.ms/u/s!AvN_8sA-Zf0djlEfoV4niIubTQee?e=cscZvo
• C.H. Crickmay suggested modifications in Davisian model of ‘geographical cycle’ in 1933 and described the process of panplanation to be more powerful and effective than Davis’ process of peneplanation in the evolution of landforms. According to Crickmay the end product of the cycle of erosion would be panplain and not the peneplain.
• L.C. King proposed a new cycle of erosion named as ‘the cycle of pediplanation’ to explain the charac teristics and evolution of landforms of arid and savanna regions of Africa as he found Davisian model of geographical cycle unfit to explain the landforms of the aforesaid regions.
• J.C. Pugh (1966) and M.F. Thomas (1966) propounded the concept of ‘savanna cycle of erosion’ to account for the development of landforms of semi-arid savanna regions of Africa.
• A.N. Strahler (1950), J.T. Hack (1960) and R.J. Chorley (1962) rejected the evolutionary concept of landform development as advanced by W.M. Davis and his followers and pleaded for the concept of ‘time-independent landforms’ instead of Davisian concept of ‘time-dependent landforms’ and advanced the concept of ‘dynamic equilibrium model’ of landform development.
• Recently, ‘tectono geomorphic model’ of Marie Morisawa (1975, 1980), ‘episodic erosion theory’ of S.A. Schumm and R.W. Lichty (1965) etc. have been suggested to explain the landform development. These models are, in fact, modified forms of Davisian model of landform development.

Question 63

Fundamental Concepts of Geomorphology: Concept 1: active always though not always with same intensity?

Answer 63

1. Hutton’s concept ‘that physical processes were always active with same intensity throughout geological periods’ is erroneous and confusing.
   
   1. For example, glaciers were more active during Carbonifenous and Pleistocene periods than other processes. At the same time, they were more active during aforesaid periods than the present glaciers.
   2. Similarly, vulcanicity was not uniformly active throughout geological periods. It was more active during Cretaceous period than today. The Cretaceous lava flow was so wide spread that extensive lava plains and plateaus were formed in almost all of the continents including basaltic lava flow over Peninsular India.
   3. The mountain building was confined to certain periods only e.g. pre-Cambrian, Caledonian, Variscan (hercynian) and Tertiary periods of mountain building.
2. The temporal variations in the magnitude of operation of processes are because of climatic changes and there are definite evidences for several phases of climatic changes during past geological times. Thus, the distributional patterns of different climatic types have registered spatial shiftings during geological past.
   
   1. For example, some areas, which are presently characterized by humid climate and dominance of fluvial process, have been dominated by dry climatic conditions and aeolian process. Similarly, some of the present dry desert areas have been humid regions in the past.
   2. For example, the fossils of coal found in Great Britain are indicative of vegetation community of equatorial climate, which forcefully proves that Great Britain, which enjoys humid temperate climate at present, was characterized by hot and humid equatorial climate during Carboniferous period when the present-day tropical areas were dominated by glacial climate.
   3. ample evidences are available to elucidate several phases of climatic changes in India. There is presence of glacial boulders and boulder clay just below the Talchir coal seams in Orissa. Most of the coal seams of India were formed during Gondwana period, which means before the formation of Gondwana system of rocks (sedimentaries including coal), the regions having coals in India were glaciated. The coal seams overlying glacial boulder indicate the prevalence of hot humid climate.
3. It is, thus, obvious that geomorphic and tectonic processes were active in all the geological periods and their mode of operation was the same as today (eg. rivers formed their valleys through vertical and lateral erosion in the past in the same manner as they are forming their valleys to day, sea waves shaped coastal areas in the same manner as they are doing today, the glacial movement and erosion was controlled by the same laws and principles during Carboniferous and Pleistocene periods as they are controlled today etc.) but the intensity of erosional and depositional works differed temporally.

Question 64

Fundamental Concepts of Geomorphology: Concept 2: statement? Headings?

Answer 64

Geologic structure is a dominant control factor in evolution of landforms and is reflected in them.

1. Intro
2. Lithology
3. arrangement of rocks
4. rock characteristics

Question 65

Fundamental Concepts of Geomorphology: Concept 2: intro?

Answer 65

1. The above concept demonstrates imposing influence of geological structure on primary and secondary landforms
2. W M. Davis included structure’ in his ‘trio’ namely structure, process and time, as important controlling factors of landscape development through his postulate that landscape is a function of structure, process and time but he gave more importance to ‘time’
3. A few usages like ‘structural geomorphology, “vol canic landforms,” arenaceous landforms”. ‘argillaceous landforms, calcareous landforms. Igneous landforms, metamorphic landforms etc. clearly demonstrate the views of a host of geomorphologists about strong control of geological structure and lithological characteristics on morphological characteristics of a region.
4. Even the modern geomorphologists like J.T. Hack. RJ. Chorley, S. Schumm, D.E. Sugden etc. have clearly outlined influences of geological struc ture on landforms.
5. This does not mean that geological structure is always and only dominant control factor in the evolution of landforms as sometimes exogenetic (denudational) processes become so effective and dominant that they overshadow the control of geological structure.

If structure is used in narrow sense of the term then it includes only deformation and arrangement of rocks due to earth-movements (endogenetic forces) but if this term is used in wider sense then structure includes (i) nature of rocks (lithology, meaning rock types). (ii) arrangement of rocks (widely known as structure) and (iii) rock characteristics. Here, ‘structure’ is used in wider sense of the term so as to demonstrate influences of all the aforesaid aspects of geological structure and landforms.

Question 66

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks?

Answer 66

• Lithological aspect of geological structure includes types of rocks (e.g. igneous, sedimentary and metamorphic groups of rocks).
• Lithological characteristics have greater significance in geomorphology because these determine and control the evolution of landforms and nature of landscape. Considering this fact S. W. Wooldridge and R.S. Morgan aptly remarked, ‘rocks whether igne ous or sedimentary, constitute on the one hand the manuscripts of the past earth-history, on the other, the basis for contemporary scenery’.
• In fact, different types of rocks differ considerably as regards their composition and chemical characteristics and hence weathering and erosional processes act upon them at varying rates thus giving birth to variations in landform characteristics.
• The relatively hard rocks (most of igneous and metamorphic rocks) give birth to bold topography. Sometimes, the influence of some rocks on geomorphic features is so dominant that the resul tant landscape is named after the rock group or individual rock e.g. granitic landforms, karst or limestone landforms, chalk landforms etc. The association of few rocks and their topographic expressions (landforms) may be examined to elucidate the con cept in question.

Question 67

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks: Igneous Topography?

Answer 67

Variations in structure and composition of igneous rocks of a particular area exert strong influence on the genesis, development and nature of landscape. Further, intrusive (e.g. granites) and extrusive (e.g. basalt) igneous rocks influence land form characteristics differently depending on their degree of relative hardness.

• Massive lava flows over extensive areas result, after cooling and consolidation, in the formation of lava plateaus the surfaces of which are least affected by fluvial erosion because the drainage is conducted underground by the joint systems, permeable ash and flow cavities, but deep weathering of basalt (especially where closely jointed in the humid tropics) and areas of poorly welded tuffs may lead to considerable piecemeal reduction of volcanic plateau by erosion’ (Chorley et. al. 1985) but the rivers, which develop over the basaltic plateaus and are subsequently fully established, resort to vigorous valley deepening through active downcutting with the result the extensive basaltic plateau is segmented into numerous smaller plateaus characterized by flat tops and steep slopes on all sides. Such features are called as mesas and buttes. Basaltic plateaus and plains give birth to picturesque land scapes after continued weathering and erosion. Very deep and long gorges and canyons have been formed by the source segments of the Saraswati and the Krishna rivers through their vigor ous vertical erosion in the massive and thick basaltic covers of Mahabaleshwar plateau (about 100 km south-west of Pune). Similarly, the Ullahas river has entrenched a very deep gorge in the basaltic plateau near Khandala (hetween Bombay and Pune). The Yellowstone river has dug out a large canyon in the Columbian lava plateau of the USA
• If the sills are intruded in the tilted or inclined sedimentary layers and if they are more resistant than the surrounding sedimentary rocks, the latter are eroded more than the former and thus resistant sills project above the general ground surface as cuestas and hogbacks (fig. 21).
• Granitic rocks when subjected to exfoliation or onion weathering give birth to domeshaped landforms known as exfoliation domes. Several exfoliation domes of granite-gneisses are seen over the Ranchi plateau, for example, Kanke Dome near Ranchi city, a group of gneissic domes near Buti village (near Ranchi city).
• Batholithic Domes
• Messas and Buttes
• The grantic rocks having rectangular joint patterns are weathered and eroded along the inter faces of their joints and thus smaller tables or blocks are separated by the eroded narrow clefts developed along the joints. Such granitic topography develops rectangular drainage pattern
• The igneous rocks having columnar joints give birth to hexagonal landforms after weathering and erosion.
• Scoria and ash cones when subjected to fluvial erosion develop radiating rills and gullies whereas strato-valcanic cones, after prolonged erosion, are characterized by numerous radiating valleys known as ‘barrancas’. The valcanic pipe filled with breccia is exposed after prolonged erosion above the ground surface and is called diatreme. Shiprock (fig. 12.8) of New Mexico (USA) is fine example of diatreme which projects 515m above the surround ing surface composed of sedimentary rocks. If magma is intruded as sills into inclined sedimentary beds of weak resistance then the sedimentary beds are eroded and the sills being resistant project above the ground surface.

Question 68

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks: Bathlithic Domes?

Answer 68

Massive granite batholiths, when exposed to the earth’s surface due to removal of superincumbent load of overlying rocks through continued erosion, become interesting landforms. These dome-shaped hills project above the general surface Such exposed granite-gneissic domes are very often found on Ranchi Plateau The granitic batholiths were intruded in the Dharwarian sedimentares during Archaean period. After a long period of prolonged subaerial erosion, the Dharwarian sedimentares have been removed and the batholiths regionally known as Ranchi Batholiths, have been exposed above the ground surface. Murha Pahar near Pitahuria village located to the NW of Ranchi city, is a typical example of exposed grantic-gneissic batholithic domes. These exposed batholithic domes have suffered intense fracture because of the removal of superincumbent load of Dharwarian sedimentaries and hence resultant massive joints have been re sponsible for the development of different types of ‘tors’. Extensive granitic domes of Yosemite Park, Sierra Nevada, Stone Mountain of Georgia (U.S.A.) and Sugar Loaf of Rio de Janeiro (Brazil) are other examples of such granitic domes which have been formed due to unloading of superincumbent load (sedimentaries) consequent upon prolonged erosion.

Question 69

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks: Mesas and Buttes?

Answer 69

• The differential erosion of the basaltic ‘cap rocks’ (fig. 2.2) produces interesting features like mesas and buttes.
  
  • Mesa is a Spanish word meaning thereby a table. Mesa, in fact, is such a hill which is characterized by almost flat and regular top-surface but by very steep slopes (wall-like) from all sides.
  • When mesas are reduced in size due to continuous weathering and erosion, they are called buttes.
  • Messas are locally called as ‘Pats’ or ‘Patland’ on the Chotanagpur plateau of south Bihar. Jamira pat, Netarhat Pat, Bagru pat, Khamar pat, Raldami pat. Lota pat etc. are typical examples of lava-capped messas of the western Chotanagpur High Lands.
  • Mahabaleshwar plateau and Panchgani plateau (of the Western Ghats, Maharashtra) are characteristic representatives of well developed basaltic mesas.
  • Grand Mesa and Raton Mesa of the state of Colo rado, USA, are typical examples of extensive mesas. Grand Mesa rises more than 1500m (5,000 feet) higher than the surrounding ground surface.
• Sometimes magma is injected in a vertical columnar form in the sedimentary rocks. The upper portion of vertical column of magma appears as butte when the overlying rocks are eroded down. Such butte is called as ‘volcanic butte’ (fig. 2.3).
• mesas and butles are confined not only to basaltic plateau but these have also been found over sandstone rocks where these overlie weak shales and siltstones. Morcha pahar (Hazaribagh plateau, Bihar, India) is a fine example of sandstone capped mesa. Similarly, Bhander plateau (M.P., India) having Vindhyan sandstones over weak shales and siltstones is an example of extensive mesa. It may, thus, be concluded that the development of mesas and buttes is no doubt lithologically control led but these are not confined to a particular rock type. They may be formed through active fluvial erosion in humid and subhumid climate whenever relatively resistant rock overlies weak rock.

Question 70

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks: Tors?

Answer 70

Well jointed granitic rocks give birth to very peculiar landforms such as tors which ‘are piles of broken and exposed masses of hard rocks particu larly granites having a crown of rock blocks of different sizes on the top and clitters (trains of blocks) on the sides. The rock-blocks, the main components of tors, may be cuboidal, rounded, an gular etc. in shape. They may be posted at the top of the hills, on the flanks of the hills facing a river valley or on flat basal platform’ A few alternative hypotheses of tor formation have been put forth e.g. pediplanation theory of L.C. King, deep basal weathering theory of D.L. Linton, periglacial theory of J. Palmer and R.A. Neilson. two-stage theory of J. Demek, glacial theory of R. Dalh etc. but there is no unanimity among the exponents because tors are not confined to a particular rock type and climate as tors have been found over granites (even basalt), sandstones, limestones etc. right from humid tropical to periglacial climate.

Question 71

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks: Sedimentary Topography?

Answer 71

• Sometime, the con trol of a particular sedimentary rock on landform characteristics is so dominant that particular rock is prefixed with geomorphology e.g. ‘limestone geomorphology’ or karst geomorphology etc.
• Sandstones having silica cementation are resistant to chemical weathering and hence give birth to bold topography and development of low drainage density while sandstones cemented by ferrous contents are subjected to rapid rate of oxidation and fluvial erosion and hence give birth to undulating and rolling terrain.
• The argillaceous rocks e.g. clay and shale are less resistant to erosion and thus low relief is associated with them. Argillaceous rocks respond differently in humid, arid and semi arid environment e.g.
  
  • in humid regions these are characterized by low relief, low to gentle slope angles (less than 8°), moderate drainage density., dendritic drainage pattern, convexo-concave hills;
  • subhumid and semi-arid regions having clay-shale rocks are characterized by the development of badland topography with high drainage density (due to numerous rills and gullies) and subdued reliefs, the gully valleys having steep valley sides (30°-60° and sometime 70°-80°) are separated by narrow ridges.
• Calcareous rocks (e.g. limestones, dolomites and chalk) are subjected to solution under humid conditions and give birth to solution holes and de pressions of varying shapes and dimensions (e.g. sink holes, swallow holes, dolines, polje, uvala etc.), underground solution networks (caves and associated features), disorganized and poor surface drainage etc. The landforms developed on carbonate rocks are collectively called as karst topography. In humid tropics two special types of karstic topography have been identified e.g. cone karst, in the cockpit country’ of Jamaica and Cuba, characterized by steep sided rounded hills, and tower karst, in monsoon land of China and Vietnam, characterized by isolated very steep sided (almost vertical) narrow but high pillars (upto 300m).
• Wherever sandstones overlie shales and siltstones majestic mesa and butte are formed and escarpments are crowned by stupendous steep scarps (e.g. Rewa escarpments, Bhander escarpments, Rohtas plateau escarpments etc. where Vindhyan sandstones lie over shales and siltstones).

Question 72

Fundamental Concepts of Geomorphology: Concept 2: Lithology or nature of rocks: Metamorphic Topography?

Answer 72

Unlike sedimentary and igneous rocks metamorphic rocks are not pronounced in the develop ment of landforms because these (e.g. quartzite, slate, schist, gneiss etc.) have uniform resistance to erosional processes though the process of meta morphism ‘converts rocks of lower resistance (e.g. shale and sandstone) to those of higher resistance (eg. slate and quartzite).

Although metamorphic rocks generally present more resistance to erosion than do their sedimentary counterparts, it is not easy to identify a separate class of distinctly metamorphic landforms

Quartzitic sandstones when lie over shales and siltstones give birth to stupendous escarpment characterized by upper free face and rectilinear segment and basal concave pediment section. Quartzites are on an average resistant to mechanical and chemical weathering and produce bold topography having very high reliefs.

Slates are more succeptible to erosion and are associated with subdued reliefs while resistant schist rocks produce highland topography.

Gneissic rocks form domes and tors.

Question 73

Fundamental Concepts of Geomorphology: Concept 2: Arrangement of Rocks?

Answer 73

Arrangement of rocks means disposition of rock beds mainly of sedimentary rocks due to deformation processes. Sedimentary rocks are generally deformed due to isostatic, tectonic and orogenetic mechanisms into folded, faulted, domed, homoclinal (uniclinal) structures etc.

Horizontal disposition of sedimentary beds denotes least deformation but these may be subjected to upwarping. Such geological structures exert strong influence on land form characteristics

Question 74

Fundamental Concepts of Geomorphology: Concept 2: Arrangement of Rocks: Folded Structure and Landforms?

Answer 74

• Sedimentary rock beds are sqeezed and buckled and folded into anticlines and synclines due to lateral compressive forces.
• The folded structure ranges from simple folds to complex folds (ie, recumbent folds depending on intensity of compressive forces)
• Simple folded structure is characterized by sequence of anticlines and synclines and in the initial stage trellis drainage pattern evolves over such structure. Such drainage pattern is characterized by the devel opment of consequent, subsequent, obsequent and resequent streams.
• The region of folded structure when subjected to continued fluvial erosion for longer period experiences the process of inversion of relief wherein original anticlines (due to more erosion) are eroded down and become anticlinal valleys where as synclines (due to less erosion) become synclinal ridges

Question 75

Fundamental Concepts of Geomorphology: Concept 2: Arrangement of Rocks: Faulted Structure and Landforms?

Answer 75

A fault is a fracture in the crustal rocks wherein the rocks are displaced along a plane called as fault plane. In other words, when the crustal rocks are displaced due to tensional movement caused by the endogenetic forces along a plane, the resultant struc ture is called a fault. Different types of faults are created due to varying directions of motion along the fault plane e.g. normal faults, reverse faults, lateral or strike-slip faults, step faults, transform faults etc.

Different fault types produce, after erosion, landforms of varying characteristics.

Take the case of normal fault . Such fault scarps after prolonged erosion produce differ ent types of erosional landforms e.g.

(a) consequent faultline scarp is formed due to erosion of weak rocks of downthrown blocks. Such fault scarps are oriented towards the direction of original fault scarp (fig. 2.8 (1);
(b) reverse or obsequent faultline scarp developes in opposite direction to the original fault scarp due to erosion of weaker strata of the upthrown block of the fault. Such fault line scarps are formed at much later date at relatively lower height (fig. 2.8 (3)).
(c) Resequent faultline scarp is formed due to renewed downward erosion caused by further fall in base-level of ero sion. In fact, resesequent scarps result from the reversal of obsequent scarp and it is oriented in the direction of the original normal or consequent scarps but is much older than the latter

Question 76

Fundamental Concepts of Geomorphology: Concept 2: Arrangement of Rocks: Domed Structure?

Answer 76

Domed structure results either due to upwarping of crustal surface effected by diastrophic force or due to intrusion of magma into surficial rocks.

The superincumbent material is removed due to prolonged erosion and the underlying structure is ex posed to the surface and few typical features like cuesta, hogback and ridges are formed.

Domes formed due to upwarping are characterized by the development of radial or centrifugal drainage pattern having a set of sequent streams which follow the slope gradient e.g. consequent, subsequent, obsequent and resequent streams (fig. 2.9).

Question 77

Fundamental Concepts of Geomorphology: Concept 2: Arrangement of Rocks: uniclinal/homoclinal Structure?

Answer 77

Homoclinal structures are those which represent inclined rock strata at uniform dip angle caused by general regional tilt.

‘These structures are formed in two main ways, either by the uplift of a sequence of off-lapping coastal plain sediments or as part of one limb of a large dome or fold’

Such structures involve both hard and soft rocks and sometimes there are alter nate bands of soft and resistant rocks and hence these are subjected to differential erosion with the result rivers form their valleys along soft rocks giving birth to the formation of strike vales while resistant rock beds are less eroded and hence become lines of asymmetrical hills known as cuesta having one side of steeper scarp slopes while other side represents gentle slope.

Homoclinal structure formed due to general tilting of sedimentary beds of coastal plains and retreat of sea water presents ideal condition for the development of consequent and subsequent streams.

The consequent streams drain seaward across resistant and weak rock beds alike but the lateral subsequent streams develop on the less resistan rocks. Thus, lines of asymmetrical cuesta features having steeper landward facing scarp slopes and gentler seaward facing dipslopes are formed parallel to the coast lines

Question 78

Fundamental Concepts of Geomorphology: Concept 2: Arrangement of Rocks: Horizontal Structure and Landforms?

Answer 78

If the regional sedimentary formation has developed well defined horizontal beds of resistant rocks, say sandstones, then after fluvial erosion tabular topography is formed.

The uplifted horizontal thick beds of relatively resistant rocks (e.g. sandstones) lying over shales and siltstones, when subjected to erosion from all sides, produce isolated flat-topped hills known as mesa (of large size) and butte (of smaller size). Such numerous features have developed over Rewa and Bhander plateau (M.P.). In fact, Bhander plateau having massive sandstone capping over shales and siltstones of Vindhyan formation is itself an example of very extensive mesa while a few smaller mesas have developed around Bhander plateau (fig. 3.8). Look hill in Jawa block of Rewa district (M.P.) is fine example of mesa capped with Vindhyan sandstone overlying shales.

The horizontal structures having alternate bands of sandstones and shales or sand stone-limestone - shale, are subjected to differential erosion and give birth to step-like scarps and bench topography (structural benches). The Grand Canyon (Colorado, U.S.A.) having horizontal beds of alternate bands of sandstone, limestone and shale presents a picturesque view of well pronounced structural benches flanking the deeply entrenched canyon of the Colorado river. Even horizontally dis posed basaltic beds of different phases of lava flow sometimes are of varying resistance and after vigorous erosion produce picturesque stepped topography (e.g. source tributaries of the Savitri and the Krishna rivers have produced Grand-Canyon-like topography around Mahabaleshwar plateau in Maharashtra). Tooth-like topography develops over resistant quartzitic sandstones whereas impervious and insolu ble resistant rock produces rounded topography.

Question 79

Fundamental Concepts of Geomorphology: Concept 2: rock Characteristics?

Answer 79

• The rock characteristics include
  
  • chemical and mechanical composition of rocks,
  • permeability and impermeability,
  • joint patterns,
  • rock resistance etc.
• Chemical composition determines nature of chemical weathering of rocks which in turn determines resultant landforms.
  
  • For example, limestone composed of calcium carbonate is very much prone to intense chemical weathering under humid condition and hence running and groundwater, when acts on carbonate rocks, produces picturesque limestone landscape (karst topography). Dolomite having mag nesium carbonate as principal constituent is also readily attacked by acidulated water. Some sandstones having calcareous or ferrous cements undergo the process of chemical erosion under warm and humid climatic conditions.
  • The prolonged chemical action on some common minerals and rocks produces dif ferent kinds of clay (e.g. terra-rosa on limestone and dolomite, kaolinite on granite and gneiss, clay on chalk etc.) the thick accumulation of which on surface causes soil creep and slumping resulting in gentle rounding of the existing landscape. The resultant soil creep produces convex slope.
• Rock joints are considered to be significant attribute of rock characteristics which influence landform characteristics both at macro-and micro scales because rock joints determine permeability of rocks, their weathering and erosion and detailed shape of some landforms.
  
  • A well jointed rock being more permeable is subjected to intense chemical weathering because it allows downward movement of corroding agent (solvent water).
  • Similarly, rocks having well developed joint pattern are vulnerable to mechanical disintegration into big rock blocks.
  • A permeable rock having well developed joint system reduces surface drainage by allowing efficient down ward movement of water and hence fluvial erosion and transportation at the surface is remarkably mini mized.
  • Joint pattern also influences development of drainage pattern at least on well jointed rocks.
  • Widely jointed granites after weathering produces ‘tors’ while poorly jointed rocks like besalt are chemically decomposed enmass.
• ‘Permeability refers to the capacity of a rock for allowing water to pass through it. A prime factor determining the degree of permeability is the presence of bedding planes and joints, but in some instances porosity can promote and enhance perme ability. Porosity refers to the presence of small gaps between the constituent mineral particles of a rock’ (R.J. Small, 1976).
  
  • Highly permeable rocks disfavour erosion as these allow more efficient percolation of water and hence form high relief topography e.g. high plateaus, escarpments and ridges (for example, sandstones and limestones) while
  • impermeable rocks (e.g. clay and shale), which are mechanically weak, discourage percolation of water and hence are more readily eroded and produce undulating vales and lowlands.
• Rock hardness is always considered in relative sense because a particular rock may be resis tant to weathering and erosion in certain environ mental condition while the same rock may be less resistant or weak in other environmental conditions.
  
  • For example, limestone becomes weak rock in humid climatic conditions because of active dissolution of rock but the same rock becomes relatively resistant in hot and dry climate due to absence of water.
  • Normally, less resistant rocks (e.g. clay, shale) are more rapidly eroded and give birth to lowland while resistant rocks produce bold topography due to less crosion.

Question 80

Fundamental Concepts of Geomorphology: Concept 3: statement? Headings?

Answer 80

Geomorphic processes leave their distinctive imprints upon landforms and each geomorphic process develops its own characteristic assemblage of landforms.

1. Intro

Question 81

Fundamental Concepts of Geomorphology: Concept 3: intro?

Answer 81

According to W.D. Thornbury geomorphic processes include all those physical and chemical changes which affect earth’s surface and are involved in the evolution and development of landforms of varying sizes and magnitudes,

Question 82

Fundamental Concepts of Geomorphology: Concept 3: different types of processes at work in geomorphology?

Answer 82

1. Exogenous
   
   1. exogenetic processes are called as grada tional or planation processes because these are continuously engaged in removing vertical irregu larities created by endogenetic processes through denudational mechanism (including both weather ing and erosion) and depositional activities.
   2. The planation work of the earth’s surface irregularities is accomplished through Degradation (weathering, erosion) and Aggradation (Deposition)
   3. Driven by solar energy
2. Endogenous
   
   1. caused by thermal conditions of the interior of the earth and varying physical and chemical properties of the materials of which the earth’s interior has been composed of,
   2. introduce vertical irregularities on the earth’s surface and create various suites of habitats for biotic communities.
   3. The significant endogenetic or hypogenous processes include diastrophic, seis mic and volcanic activities.
3. Extra-terresterial: fall of meteorites
4. Anthropogenous

Question 83

Fundamental Concepts of Geomorphology: Concept 3: geomorphic processes leave their distinctive imprints?

Answer 83

• Generally, endogenetic processes introduce vertical irregularities on earth’s surface while exogenetic processes works towards planation
• Different Erosional processes have different impacts
  
  • Corrosion, involving chemical dissolutoin of carbonate rocks leave behind characteristic landforms. Absence of surface streams and pre-dominance of caves and caverns makes it distinct from other erosional processes like abration.
  • Aeolian abrasion leaves a distinctive imprint through processes of ‘sandblasting’. Aeolian abrasion is minimum at ground level in contrast to that by river water which engages in pothole drilling along river bed as well as lateral erosion that leads to widening of river valley.
  • Deflation by wind leads to unique features like ‘blowouts’
• Congelifraction (frost weathering), congelifluction (soil creep), frost heave (bulging and subsidence), nivation (snow patch erosion) etc. are characteristic weathering and transportation mecahnisms performed by periglacial processes. The mechanism of erosion, though very slow and insignificant, by periglacial processes is cryoturbation.
• Different processes of transportation also vary in various aspects. eg. transportation by streams is unidirectional while that by sea waves is bidirectional and aeolian transportation is multi-directional.

Question 84

Fundamental Concepts of Geomorphology: Concept 3: process characteristic landforms?

Answer 84

Because of distinctive characteristics the landforms produced by one particular process may be differentiated from those produced by other proc esses. For example,

alluvial cones and fans, flood plains, gorges and canyons, natural levees, river meanders, and deltas are indicative of the work of fluvial process (streams) while

solutional holes and depressions (sink and swallow holes, dolines, polje, uvalas etc.), limestone caves, stallectites and stalagmites are the products of the erosional and depositional works of groundwater on carbonate rocks.

Sand dunes indicate the depositional work by winds

moraines, drumlins, eskers etc. and U-shaped valley with hanging valley, cirque, aretes etc. denote the product of glacial process whereas

patterned ground (stone circles, stone nets, stone polygons etc.), pingo, thermokarst, solifluctate lobes and terraces, stone glacier, blockfields, altiplanation terraces,nivation hollows etc. are the exclusive responses of periglacial processes.

Question 85

Fundamental Concepts of Geomorphology: Concept 3: A landform may be a result of different types of processes and agents: example (can be used in schematic)?

Answer 85

• Plains
  
  • flood plain- by flood deposition
  • peneplain by fluvial processes
  • karst plain by GW
  • pediplain by scarp retreat and pedimentation in semi-arid areas
  • panplain - by coalescenceof flood plains caused by lateral erosion by fluvial processes
  • etchplain: by etching and washing of debris by streams in Savanna region
  • alluvial plain: deposition by streams
  • outwash plain: due to fluvio-glacial action
  • cryoplain- due to cryoplanation
• Ridge
  
  • anticlinal ridge: tectonic
  • synclinal ridge- erosion by streams
  • hogback ridge- tectonic and erosional
  • beach ridges- deposition by sea waves
  • morainic ridge- deposition by glaciers
  • nivation ridge- deposition by peri-glacial processes

Question 86

Fundamental Concepts of Geomorphology: Concept 3: conclusion?

Answer 86

very few landforms are of mono-process origin because most of the landforms have been developed by more than one one processes i.e. they are of poly-process origin as different geomorphic processes seldom operate in isolation.

For example, even in periglacial environment different geometrical patterns (very commor.y called as patterned ground having definite geometrical patterns such as circle, net, polygon, stripe etc.) are formed due to combined actions of frost heave and solifluction whereas involutions, hummocks and pingo are formed by congelifraction (frost weathering) and altiplanation landforms are the result of combined actions of solifluction, nivation, frost heave and congelifraction.

Question 87

Fundamental Concepts of Geomorphology: Concept 4: statement?

Answer 87

As the different erosional agencies act on the earth’s surface there is produced a sequence of landforms having distinctive characteristics at the successive stages of their development.

Question 88

Fundamental Concepts of Geomorphology: Concept 4: intro?

Answer 88

The present concept is related to one of ‘trio of Davis’ (landscape is a function of structure, process and time). The stated concept is based on the concept of ‘cyclic time’ which involves long geological period of millions of years and larger spatial areas. It may be pointed out that Davis used ‘time’ as a process’ rather than ‘an attribute’ of landscape development wherein he envisaged sequential changes in landforms through time.’ ‘For Davis, the concept of evolution implied an inevitable, continuous and broadly irreversible process of change producing an orderly sequence of landform transformation, wherein earlier forms could be considered as stages in a progression leading to later forms. By this model, time became not a temporal frame work within which events could occur, but a process itself leading to an inevitable progression of change’

Thus, following Davis there is progressive change in landform characteristics with the passage of time.

Question 89

Fundamental Concepts of Geomorphology: Concept 4: David’s Cycle of Erosion?

Answer 89

Davis’ model of cycle of erosion is based on the concept of ‘low-entropy closed system’ wherein initial potential energy in the closed system is provided by initial rapid rate short-period upliftment of landscape.

With the passage of time and continuous erosion there is equal distribution of energy in the geomorphic system so that all components of the system are characterized by equal energy levels and hence in the absence of difference in the energy levels of different components of the system, the state of maximum disorder and hence maximum entropy is achieved wherein no further work is performed because there is no energy flow and the ultimate result is the development of peneplain.

Though this concept of Davis (closed geomorphic system characterized by evolutionary changes in the landform geometry) is subject to severe criticism but ‘for Davis, each stage or his cycle was associated with declining potential energy as the relief was worn down, and each stage was characterized by an assemblage of landforms (i.e. valley-side slopes, drainage patterns etc.) having geometries appropriate to the local potential energy expressed by the difference in level between the land surface (ridge crest or top of water divides) and some, lower elevation (base level, valley floor) towards which

Diag: https://1drv.ms/b/s!AvN_8sA-Zf0djj4N2XqP8QP2xRtN?e=JKhQvw

Question 90

Fundamental Concepts of Geomorphology: Concept 4: commentary?

Answer 90

• Davisian Model of sequential changes in landforms is possible only in low entropy closed geomorphic system but the geomorphic systems having different landform assemblages are open systems wherein there is continuous input of potential energy (through upliftment of landscape, plate tectonic theory has demonstrated continuous tectonic activities), of kinetic energy (through precipitation and channel flow), of thermal energy (through insolation from the sun) and of chemical energy (through disintegra tion and decomposition of rocks) and there is con tinuous export of energy and matter out of the system and hence the geomorphic system tends to be in equilibrium condition. Thus, the Davisian concept of sequential changes of landforms through succes sive stages is not tenable.
• Moreover, it is argued that the life cycle of landform development cannot be equated with hu man life cycle because the time span of three stages of the latter (youth, mature and old) is almost fixed and one stage changes to the next stage after certain time period but this is not possible in the case of landscapes because a region having weak and less resistant rocks is quickly eroded down and youth stage advances to mature stage within shorter period of time but if the region is characterized by hard and resistant rocks then the period of youth stage is lengthened and change from youth to mature stage is much delayed. This is why W. Penck pleaded for the rejection of Davis’ concept, ‘landscape is a function of structure, process and time (stage)’, and postu lated the concept that, ‘landforms reflect the ratio between the intensity of endogenetic processes (i.e. rate of upliftment) and the magnitude of displace ment of materials by exogenetic processes (the rate of erosion and removal of weathered and eroded materials)’.
• It may be further argued that each stage of geomorphic cycle does not have same time-period. Further, if the landscape development in different regions is passing through similar stage (say youth stage) it does not mean that the time-period of similar stage is the same in all regions. If two regions are characterized by same stage of landscape devel opment the landform characteristics in both the regions may be similar but not the same.

Question 91

Fundamental Concepts of Geomorphology: Concept 5: Statement?

Answer 91

Geomorphic scale is a significant parameter in the interpretation of landform development and landform characteristics of geomorphic systems

AND

Landscape is a Function of time and space

Question 92

Fundamental Concepts of Geomorphology: Concept 5: intro?

Answer 92

‘Inclusion of time dimension is necessary because periods of time may be necessary for a certain process or assemblage of processes acting upon particulate materials to produce a spe cific form’

geomorphic investigation requires study of different geomorphic processes (both mode and rate of operation) and related landforms of a spatial unit over definite time

Question 93

Fundamental Concepts of Geomorphology: Concept 5: Landscape is a Function of Time and space?

Answer 93

• Concept 6: KT Gregory in 1977 postulated that “A simple Geomorphological equation may be envisaged as a vehicle for the explanation of landforms as follows: F=f(PM) dt ”. As shown, P (processes) and its effect on M (Geological material) work over a period of time to have a geomorphological effect on the landforms.
• Generally, no perceptible change may occur in the morphological features during short period because either the force exerted by the processes may not be enough to introduce significant change or the processes might have not operated for desired sufficient length of time.
• Any change in the rate of operation of geomorphic process is supposed to bring corresponding change in the landforms, ‘some times the response is instantaneous, as when a large flood passes through a channel. At other times the response may be quite slow or there may be ‘dead time’ when nothing happens to landforms to reveal the change in the process’
• eg. CLimate change and global warming is a result of increase in global temperatures. While global temperatures has fluctuated by itself in th past resulting in many glacial and inter-glacial cycles, it is the unprecedented rate of increase of global temperature, as a result of anthropogenic factors, that is a cause of concern.
• process centric theories like Davis’ Cycle of erosion considers time (stages) as a determinitive factor for landfrom development process. For example, youth stage is necessarile linked to rapid rate of valey deepening, creation of rapids, waterfalls and canyons, while old stage is characterised by peneplain like landscape.
• On the other hand, advocates of ‘equilibrium’, consider landscape development on basis of relationship betn driving force and resisting force with no clear demarcation of stages.
• The role of time in various gromorphological processes can be studied on various scales- macro, meso and micro temporal scales. For geomorphic evolution and and interpretation, temporal scales are considered at three resolution levels- cyclic time, graded time and steady time. ‘It will be seen that time can be considered as the most significant independent variable in landform studies, or regarded as of relatively little signifi cance, depending upon the time-span involved
  *

Question 94

Fundamental Concepts of Geomorphology: Concept 5: “Geomorphic scale is a significant parameter in the interpretation of landform development and landform characteristics of geomorphic systems”?

Answer 94

In the process of Geomorphic investigations, both gemorphological processes and landforms are considered at various levels of spatial and temporal resolutions.

In 1965, Schumm and Lichty argued that the kind of model we construct for the study of landform development depends upon the length of the time-span we have in mind’. ‘At different scale resolution lev els, which are mapped out according to our aims and abilities, different problems are identified, different types of explanation are relevant; different levels of organization are appropriate; different variables are dominant, and different roles of casue and effect are assigned’

Geomorphic scales can be of two types- Time scale and spatial scale.

The scale level resolutions depend on the objectives of study. For example, if the evolutionary phases of landscape development over long period of time involving larger areas are to be reconstructed, the model of Davisian cycle of erosion involving cyclic time (millions of years) may be more apropriate but if a component of landform assemblage is to be studied, a shorter time scale would be more appropriate. It may be mentioned that conclusions derived about landform development and processes at one spatial and temporal scale may not be applicable to other scales because the influence of dominant variables changes from one scale to another scale.

Question 95

Fundamental Concepts of Geomorphology: Concept 5: “Geomorphic scale is a significant parameter in the interpretation of landform development and landform characteristics of geomorphic systems”: time scales?

Answer 95

Generally, temporal scales are considered at three resolution levels e.g, macro-temproal scale involving millions of years for the study of mega geomorphology, meso-temporal scale involving thousands of years and micro-temporal scale in volving shorter time-span involving tens and hun dreds of years.

For geomorphic evolution and inter pretation temporal scales are, alternatively, considered at three resolution levels e.g. cyclic time,graded time and steady time.

• Cyclic time: Cyclic time involves longer geological pe riod of time measuring millions of years (say 10,000,000 years) and very larger spatial areal unit measuring thousands of square kilometers of area.. This time-span involves progressive but slow change both process rate and landforms. eg. Davisian model of cycle of erosion is based on cycle time wherein there is progressive sequential change in landforms through time
  
  • amongst Schumm and Lichty’s ten drainage basin variables, time, initial relief, geology and climate are independent variables which control landform development involv ing cyclic time-span, whereas vegetation, relief, hydrology, drainage network morphology, hillslope morphology are dependent variables
• Graded time: The time-scale having shorter period (say 100 to 1000 years), during which smaller streams or parts of big streams and individual hillslopes in a drainage network achieve graded stage of steady state equilibrium
  
  • As the time-span of landscape development is reduced the number of controlling factors (i.e. independent variables) increases and number of dependent variables decreases. For eg. beside the abovementioned 3 independent variables (excluding time), vegetation, relief and hydrology also become independent variables.
  • time and initial reliefs, which are very significant control ling variables (of landforms) in cyclic time become insignificant in the development of landforms in graded time
• Steady time: Still shorter time-span (10 to 100 years), during which a very short reach of the stream or a single slope segment (e.g. convex or rectilinear or concave segment) involving very small area reaches steady state, is called steady time in which there is balance between erosion, transport and deposition
  
  • The aforesaid seven variables (e.g. time, initial re lief, geology, climate, vegetation, volume of relief above base-level, runoff and sediment yield per unit area within the system, drainage, which are indipendent variables in cyclic and graded time plus drainage network morphology and hillslope mor phology (which are dependent variables in graded time) become independent variables and only one variable (i.e. discharge of water and sediment out of the geomorphic system (say drainage basin) be comes dependent variable.

‘It will be seen that time can be considered as the most significant independent variable in landform studies, or regarded as of relatively little signifi cance, depending upon the time-span involved

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djj_MfSWQoZVkBy5Y?e=OUVqCB

Question 96

Fundamental Concepts of Geomorphology: Concept 5: “Geomorphic scale is a significant parameter in the interpretation of landform development and landform characteristics of geomorphic systems”: spatial scale?

Answer 96

There has always been shift in the selection of ideal geomorphic unit having specific areal cover age for the study of landforms: from ‘physiographic re gions’ of N.M. Fenneman (1914) through Horton’s (1945) ‘drainage basin’ as ideal geomorphic unit to J.F. Gellert’s ‘morphotops’ or ‘morphofacies’

• Fenneman’s physiographic regions of N. America on the basis of chronology and uniformity of geological history and structural geology repre sent large spatial scale i.e. macro or mega scale and further subdivisions of major physiographic regions into smaller units involved small spatial scale i.e. meso and micro scales.
• J.F. Gellert (1982) recognized ‘morphotops’ or ‘morphofacies’ as basic units for morphological regionalization and ‘suggested a uniform shape (mor phology, morphometry), homogeneous lithological structure, uniform origin and development (morphogenesis, morphochronology) and uniform present-day processes (morphodynamics) as the characteristic features for the identification of geomorphological regional units’
• spatial scales have changed from macro or mega-scale (of earlier gemorphologists dealing with the cyclic development of landforms and denudation chronology) through meso-spatial scale to present-day micro-spatial scale (in the case of process geomorphology).

It may be mentioned that spatial scale has much significance in controlling the rate and mechanism. of operation of processes and their responses (landforms) as the areal coverages of study areas change. For example, if a small area (less than one square kilometer) of gullied zone is selected for the study of behaviour of runoff, discharge, soil erosion, sediment transport etc. during strong rainstorms associated with thunderstorm, the fluvial process is highly accelerated and the rate of erosion becomes very high becuase of maximum runoff and discharge but if the study area is a large drainage basin then the effect of strong rainstorm of short duration is ob scured as only the part of the basin is affected by high intensity rainstorms.

Question 97

Three types of equilibrium concepts of landform development?

Answer 97

• Decay equilibrium: there is progressive but slow rate of decline in form through time leading to establishment of equilibrium condition in the penultimate stage- akin to old stage-of Davisian cycle of erosion
• dynamic equilibrium: indicating a condition of forms oscillating around a moving average value but also characterized by continuous decline in form through time e.g. a river’s long profile characteized by alternate actions of erosion and deposition
• Dynamic Metastable Equilibrium: a condition of oscillation about a mean value of form which is trending through time and, at the same time, is subjected to step-like discontinuities as a threshold effect appears to promote a sudden change of form’
  
  • a condition of equilibrium at insufficient energy level wherein erosional processes act in episodic manner
  • eg. Schumm’s theory of valley deepening: effective erosion is not a continuous process rather it is episodic in nature and thus the valley floors are not continuously deepened but are reduced in discontinuous manner as periods of erosion are separated by periods of deposition of sediments to an unstable condition. In other words, the period of erosion (period of instability) is followed by period of deposition of sediments. When the sediment storage in the valley crosses the threshold value and channel gradient is steepened then the system becomes unstable and active erosion is initi ated resulting in the downcutting (excavation of deposited sediments and valley floor) of valley floor. The process continues till the sediments are flushed out and again period of deposition is initiated due to lessening of channel gradient. Thus, the valley floor becomes stepped.

Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djkB5kIvv_siuZ1kL?e=MIPRji

Question 98

Fundamental Concepts of Geomorphology: Concept 5: conclusion?

Answer 98

The post-1950 geomorphology lays more emphasis on the study of different aspects of processes on the basis of field instrumentation and laboratory experiments. This requires shorter tem poral scale (time scale) and smaller spatial unit.

Question 99

Fundamental Concepts of Geomorphology: Concept 6: statement?

Answer 99

A simple Geomorphological equation may be envisaged as a vehicle for the explanation of landforms as follows: F=f(PM) dt

Question 100

Fundamental Concepts of Geomorphology: Concept 6: intro?

Answer 100

Gregory stated (1977) that ‘morphology (F) = function of processes (P) on materials (M) over time (t)’.

According to him

• ‘morphology refers to the form of earth’s surface or landform;
• processes include the geomorphological processes associated with weathering, wind, water, ice and massmovement; and
• materials connote the rock, soil and superficial deposits upon which processes operate’

Question 101

Fundamental Concepts of Geomorphology: Concept 6: four aspects?

Answer 101

He has identified four aspects of interest wherein the equation may be studied at four levels

1. elements of equation
2. balancingthe equation
3. differentiating the equation
4. applying the equation

Question 102

Fundamental Concepts of Geomorphology: Concept 6: elements of equation?

Answer 102

It is necessary to study detailed aspects of forms (landforms), geomaterials (of which the landforms have been formed) and processes (which shape the landforms through erosional and depositional activities) independently

1. Landforms:
   
   1. Different aspects of forms (landforms) have been widely studied and given more attention right from the beginning of geomorphological investiga tions to the development of the branch of landform geography (B. Zakrzewska, 1967).
   2. Morphometric techniques have enabled geomorphologists to study different morphometric aspects (shapes, amplitude and dimension) of landforms produced by various denudational processes.
   3. Information derived from aerial photographs and satellite imageries have also enriched landform geography.
2. Geomaterials:
   
   1. Generally, geomaterials include rock types, geological structure, rock characteristics, weathered materials, surficial deposits and soils. Traditionally, geological structure includes three aspects viz. lithology or nature of rocks (igne ous, sedimentary and metamorphic rocks), arrange ment and disposition of rock beds (folded, faulted, uniclinal, domal etc.) and rock characteristics (chemi cal and mechanical composition, permeability and impermeability, joint patterns, rock resistance etc.).
3. Processes:
   
   1. include those physical processes which operate on the earth’s surface both internally and externally (i.e. endogenetic and exogenetic processes).
   2. A detailed investigation re garding three-phase work of geomorphic processes (i.e. erosion, transportation and deposition) is needed to understand the mode of origin and development of landforms of varying scales.
   3. The detailed study of exogenetic geomorphic processes (denudational proc esses e.g. fluvial, coastal, glacial, aeolian, periglacial, groundwater etc.) through field observation and instrumentation and laboratory experimentation has gained currency since 1950.

Question 103

Fundamental Concepts of Geomorphology: Concept 6: balancing the equation?

Answer 103

After the detailed investigation of form (landforms), materials and processes individually and independently, attempt is made to produce a general model of ‘form-processes-materials relationships.’

“The system approach is ideally suited to the identification of the relationships between the elements of the equation and has been instrumental in clarifying the diverse ways in which indices of materials, of process, and of form are related’The introduction of equilibrium concept has enabled the geomorphologists to envisage the landscape devel opment on the basis of relationship between processes (driving force- operation of processes) and materials (resisting force) leading to the attainment of equilibrium when driving force equals the resist ing force

Question 104

Fundamental Concepts of Geomorphology: Concept 6: differentiating the equation?

Answer 104

Differentiating the equation requires to find out ‘the way in which geomorphological systems change or adjust over time’.

refer f/c Fundamental Concepts of Geomorphology: Concept 5: Landscape is a Function of Time and space?

Question 105

Fundamental Concepts of Geomorphology: Concept 6: application?

Answer 105

The knowledge derived through the analysis of geomorphological equation at three levels is utilized for ‘estimation of the behaviour of geomorphological systems either in locations where processes have not been measured (spatial prediction) or in the future (temporal pre diction)’

This becomes the field of applied geomorphology having varying dimensions e.g. environmental geomorphology, urban geomorphology, geomorphic engineering etc

Question 106

Fundamental Concepts of Geomorphology: Concept 7: statement?

Answer 106

Complexity of geomorphic evolution is more common than simplicity.

Question 107

Fundamental Concepts of Geomorphology: Concept 7: intro?

Answer 107

While landform characteristics of a regions are explained on the basis of most dominant controlling factor on the basic premise that majority of landforms are simple and have less complex geomorphic evo lution but in reality most of the landforms are the result of poly-factor rather than mono-factor.

Most theories usually over-emphasize the significance of one factor or process at the cost of other factors. eg. cyclic theories of landforms rely on time based approach and sequential change of landforms with time; process form approach sees landscapes as representative of single dominant process; dynamic equilibrium theories rely on driving and resisting forces and their inter-play; structure form approach regards geological structure as the most dominant controlling factor while climate process form approach sees landscapes as characteristic of particular climate of the region

Question 108

Fundamental Concepts of Geomorphology: Concept 7: causes of complexity?

Answer 108

1. The present landscapes of different physiographic regions at least at macro-spatial scale (megageomorphology) are examples of palimpsest topography because these regions have experienced several phases of cycles of erosion and the landforms have evolved very slowly over long period of geological time and thus the landscapes having superimposed effects of climate and tectonic factors show evidences of poly-cyclic evolution and complexity in their general character istics. In fact, successive cycles of erosion introduce complexity in landforms. For example, most parts of peninsular India exhibit a fine example of palimpsest topography having polycyclic reliefs characterized by different erosion (planation) surfaces at different elevations.
2. The operation of several geomorphic proc esses even during a single cycle of erosion intro duces complexity in landforms. For example, though wind is the most dominant geomorphic process in warm and hot arid regions but fluvial process be comes occasionally very active when there is occa sional heavy rainfall through strong rainstorm (though very rarely). Consequently, besides aeolian landforms (eg. inselbergs, yardang, zeugen, sand dunes etc.), very interesting fluvial landforms (pediments, bajadas, playas and badland) are also formed. Similarly, besides the development of pure glacial landforms in glaciated regions, fluvio-glacial landforms (e.g. kame, eskers, outwash plains etc.) are also evolved.
3. spatial variations in landform-con trolling factors (e.g. lithology, geological structure, climatic parameters mainly temperature and pre cipitation, vegetation, soils, human activities etc.) within a physiographic or morphogenetic region introduce complexity in the landforms.
4. Tectonic events (upwarping, downwarping, upliftment, subsidence, folding, fault ing etc.) are very important factors for creating variations in landform characteristics. They cause interruptions in cycles of erosion which com plicate the landforms through rejuvenation and in tiation of new cycles of erosion.
5. Changes in base-levels of erosion caused by negative or positive changes (fall and rise) in sea levels either due to tectonic events or climatic changes (fall in sea-level or negative change due to glacial ice age or rise in sea-level due to interglacial period) are responsible for the initiation of successive cycles of erosion and hence polycyclic landforms.

Question 109

Fundamental Concepts of Geomorphology: Concept 7: Landscape classification based on complexity: intro?

Answer 109

On the basis of variations in landform characteristics Horberg (1952) divided the landscapes of the globe into five principal categories viz.

(1) simple landscapes,
(2) compound landscapes,
(3) mono cyclic landscapes,
(4) multi-cyclic landscapes, and
(5) exhumed or resurrected landscapes.

Question 110

Fundamental Concepts of Geomorphology: Concept 7: Landscape classification based on complexity: simple landscapes?

Answer 110

• Simple landscapes are those which are generally devoid of complexity and are the result of mono process acting during a single cycle of erosion.
• For example, if we take the case of a region having sedimentary rocks consisting of alternate bands of relatively resistant (sandstones) and soft rock beds (shales) and river as agent of erosion, the differential fluvial erosion will give birth to stepped landscapes.
• It may be admitted that even simple landscape is not the result of a single geomorphic process but for simplification and generalization the most dominant process is given due importance and landscape de velopment is studied in terms of most dominant process (e.g. fluvial landscapes, glaciated landscapes, periglacial landscapes, aeolian or arid landscapes etc.). For example, if the landscape of a given region is evolved due to the work of running water (river), the fluvial process undoubtedly is the most effective geomorphic agent but weathering process (corrasion) and masswasting and masstranslocation (slumping. soil creep, mud flow etc.) also play significant role. Similarly, the solution (corrosion) mechanism is most dominant denudational mechanism by groundwater in the areas of carbonate rocks but surface water (surface runoff resulting form rainfall) also helps in the evolution of landforms.
• In fact, the concept of ‘mono-process landform’ is related to the concepts of climatic geomorphology and morphogenetic regions wherein it is envisaged that ‘each climatic type (and hence the resultant geomorphic process) produces its own characteris tic assemblage of landforms’. L.C. Peltier’s classifi cation of climatogenetic landforms into nine catego ries and division of world landscapes into nine morphogenetic regions (e.g. glacial, periglacial, boreal, maritime, selva, moderate, savanna, semi arid and arid morphogenetic regions) is based on the concept of climatic geomorphology but it may be pointed out that the advocates of climatic geomorphology have not succeeded in presenting ample convincing evidences in support of their argu ments through diagnostic landforms (e.g. lateritic feature, inselbergs, pediments, tors etc.).

Question 111

Fundamental Concepts of Geomorphology: Concept 7: Landscape classification based on complexity: compound landscapes?

Answer 111

• The landscapes, produced by more than one geomorphic processes and landform controlling factors, are called as compound landscapes.
• In fact, compound landscapes are more common in reality than simple landscapes.
• The landscapes produced during Pleistocene glaciation present examples of compound landscapes as glacial geomorphic fea tures (both erosional and depositional) are found at higher altitudes while fluvial landforms (produced by rivers) are found at lower levels. Besides, aeolian features mainly depositional forms have also devel oped.
• Several examples of compound landscapes are seen in Utah, New Mexico, Arizona, Nevada etc. of the U.S.A. where volcanic cones and related vol canic landforms and lava-flow related features have developed in the fluvially originated river valleys.
• Tectonic events also introduce complexity in the landscapes. Composite fault-line scarps are such examples. Such features bear the characteristics of fault plane as well as erosional surface. Such com posite fault-line scarps are formed when fault scarp is originated due to faulting resulting in the down ward movement of down thrown block along the fault plane and subsequent erosion of lower segment of fault scarp. Thus, the upper segment is tectonically formed (due to faulting) while the lower segment is erosional

Question 112

Fundamental Concepts of Geomorphology: Concept 7: Landscape classification based on complexity: Mono-cyclic landscapes?

Answer 112

• The landforms produced in a physiographic region during a single cycle of erosion are called monocylic landforms.
• Like simple landscapes, monocyclic landscapes are less common in reality.
• Monocyclic landscapes may be possible along coastal plains provided that the coastal plains are not affected by several phases of emergence and sub mergence.
• Monocyclic landforms generally develop over volcanic cones, lava plains and lava plateaus, newly formed domes etc.
• It may be pointed out that monocyclic landscapes may be both simple and compound.

Question 113

Fundamental Concepts of Geomorphology: Concept 7: Landscape classification based on complexity: Poly-cyclic landscapes?

Answer 113

• Landscapes produced due to completion of several cycles of erosion (successive cycles of ero sion) in a region are called as poly (multi) cyclic landscapes (example of palimpsest topography).
• Most of the present-day landscapes are the examples of multicyclic landscapes which have developed during more than one cycles of erosion. It may be mentioned that landforms of older cycles are not found in their original forms because they are modi fied by succeeding phases of cycle of erosion and hence only relic features of older cycles are pre served.
• Polycyclic landscapes are identified on the basis of a few diagnostic and representative landforms e.g. valley in valley topography (multi-storyed valleys, topographic discordance), rejuvenated river valleys, uplifted peneplains, incised meanders, mek points or heads of rejuvenation etc.).
• The multi cyclic landscapes are evolved due to rejuvenation consequent upon lowering of base level of erosion either due to upliftment or negative change in sea level (fall in sea-level).
  
  • Applachian highlands of the USA present fine example of polycyclic landscapes which have developed because of three successive cycles of erosion (viz-Schooley, Harrisberg and Sommerville cycles of erosion).
  • The Damodar river valley at Rajroppa in Hazaribagh (Bihar, India) and the Narmada valley at Bheraghat (near Jabalpur, M.P.) present ideal examples of rejuvenated valleys having three-tier terraces on either side.
  • The Chotanagpur region in general and Ranchi plateau in particular represents examples of polycyclic land scapes, Hundrughagh falls on the Subarnasekhariver. or Gautamdhara falls at the confluence of the Gunga and the Raru rivers, Dassamghagh falls on the Kanchi river (a tributary of the Subarnarekha) etc. indicate heads of rejuvenation along the junction of the central and eastern Ranchi plateau (Bihar).

Question 114

Fundamental Concepts of Geomorphology: Concept 7: Landscape classification based on complexity: resurrcted landscapes?

Answer 114

The resurrected or exhumed landscapes are those which were covered with either lava flow (volcanic eruption) or sedimentation (mainly on the coastal plains) after their formation but were uncov ered at a later date due to denudational processes.

Majority of landscapes were covered with thick ice sheets during Pleistocene ice age in North America and Eurasia but these reappeared after deglaciation of ice sheets.

Many of the landscapes were buried under lava sheets in Peninsular India during Creta ceous vulcanicity and a few of them have now been exhumed due to erosion of lava cover.

Question 115

Fundamental Concepts of Geomorphology: Concept 8: statement?

Answer 115

Little of the earth’s topography is older than Tertiary and most of it no older than Pleistocene

Question 116

Fundamental Concepts of Geomorphology: Concept 8: arguments in support?

Answer 116

• It is argued by many geomorphologists, like GH Ashlay (1931), that most of the present-day landforms are the result of geomorphic processes which operated in the Tertiary and Quaternary times as the landforms older than Tertiary have been either obliterated by the dynamic wheels of denudational processes or have been so greatly modified that they have lost their original shapes and cannot be properly and accu rately identified. However, this school is burdened with Pleistocene-centric bias.
• The advocates of this concept (aforesaid) argue that pre-existing earth’s surface was greatly affected and modified by global Tertiary orogeny (formation of Alpine-Himalayan chains, Rockies, Andes, Atlas, Island arcs and festoons of east Asia etc.) and related rejuvenation of existing cycles of erosion and initiation of new cycles resulting in the origin of new sets of landforms world over.
• The Quaternary epoch experienced global climatic change and Pleistocene ice age comprising four glacial periods (Gunz, Mindel, Riss, Wurm in Europe and Nebraskan, Kansan, Illinoin and Wisconcin in. N. America) and alternated by four interglacial periods (warm period) obliterated and modified nearly all of the pre-existing landscapes in most of the regions of North America and northern Eurasia as the advanc ing ice sheets filled up the lowlying areas and lowered and rounded sharp peak and hills. The retreating ice sheets left morainic ridges and glacial lakes were formed in N.America and Europe
  
  • For example, ‘In Triassic time England was largely a desert, as was Scotland in Torridonian (Precambrian) time. In contrast, South Africa, India and Australia had gla cial climates in Permo-Carboniferous time’
• Himalayan orogeny began either during late Cretaceous period (Mesozoic era) or Eocene period (Tertiary) but it was not complete until Pleistocene period but most of the topographic details were carved out during Quaternary epoch by the fluvial processes. The Himalayas are characterized by young and rejuvenated landforms e.g. deep. long and narrow valleys (gorges and canyons), three paired terraces, waterfalls and rapids etc.
• Further, the Himalayan orogeny also effected topographic features of the Chotanagpur.
  
  • Tertiary epoch registered three phases of upliftment and hence interruptions in fluvial cycles of erosion occurred several times mainly in Palamau uplands and Ranchi Plateau.
  • The marginal areas of the Ranchi plateau (including ‘paltands’) characterized by waterfalls (Hundrughagh falls, Gautamdhara or Johna falls, Dassamghagh falls, Pheruaghagh falls etc.), nick points and breaks in slopes and juvenile characters of the rivers where these descend from the escarpments, tell the story of Tertiary upliftments.
  • The formation of the Gangetic trough consequent upon the Himalayan orogeny rejuvenated the foreland of Indian peninsula which is evidenced by the presence of a series of waterfalls on the northward flowing rivers which after descending through the foreland meet the Yamuna and the Ganga rivers right from the extreme western point of the Rewa plateau (M.P.) to Rohtas plateau in the east (Bihar) e.g. Tons or Purwa falls (70 m), Chachai falls (127m), Kevti falls (98m), Odda falls (148 m, all in M.P.), Devdari falls, Telharkund falls (80m), Sura falls (120 m), Durgawati falls (80 m), Dhuan Kund falls, Rahim Kund falls (168 m) etc. (all in Rohtas plateau, Bihar).
• A major factor behind pre-dominance of Pleistocene landforms is that erosional and weathering processes, responsible for creation of most of third order landforms are largely determined by climatic conditions and hence climatic changes like glaciation and warming during Pleistocene had a greater impact

Question 117

Fundamental Concepts of Geomorphology: Concept 8: counter?

Answer 117

On the other hand, some geomorphologists also argue that the present-day landform assemblages are the examples of palimpsest topography and are the result of past (palaeo) and present processes.

Some of the relics of landforms resulting from weathering and erosional processes as a conse quence of climatic changes through geological times have been preserved. So in the thinking of time-scale we are concerned not only with the formation, but also the preserva tion of landforms. There are places where actual landforms, such as river valley systems, have been preserved for hundreds of millions of years’

eg. Sevier and Laramide orogenies, which took place in western North America during Cretaceous time. These events created the Rocky Mountains.

Two of the largest volcanic events in Earth’s history occurred during the Mesozoic.

• The Central Atlantic Magmatic Province, a huge volume of basalt, was created at the end of the Triassic during the initial rifting of Pangea. The surface area of this igneous province originally covered more than 7 million square km (about 3 million square miles), and its rocks can be found today from Brazil to France.
• At the end of the Cretaceous, another igneous province, the flood basalts of the Deccan Traps, formed on what is now the Indian subcontinent.

Question 118

Fundamental Concepts of Geomorphology: Concept 8: conclusion?

Answer 118

It may be concluded that, no doubt, the cli matic escillations and tectonic activities since Terti ary and mainly during Quaternary have so greatly modified (Pleistocene glaciation) pre-existing mor phological features that they have lost their original characteristics at least in North America and north ern Europe but many relic geomorphic features of longer geological histories are indicative of their palaeo-genesis.

Question 119

Fundamental Concepts of Geomorphology: Concept 9: statement?

Answer 119

Each climatic type produces its own charac teristic assemblage of landforms”.

Question 120

Fundamental Concepts of Geomorphology: Concept 9: Climatic Geomorphology: intro?

Answer 120

This concept is based on the basic tenet of climatic geomorphology based on the work of Von Richthofen (in China) and passarge, Jessen and Walther’s work in Africa.

The concept envisages that geomorphic processes, which shape the landscapes. are determined and controlled by climate which thus produces distinctive landscapes through processes.

The advocates of climatic geomorphology have attempted to validate the influences of climatic conditions on the evolution and characteristics of landforms on the basis of certain diagnostic landforms such as duricrusts (such as laterites, silcrete, calcrete etc.), inselbergs, pediments, tors etc.

It is argued that climatic parameters control landscape development directly and indirectly. Cer tain climatic parameters such as temperature and humidity (precipitation) directly control weathering and erosional processes while indirect influence of climate on landforms is through vegetation and soils.

The climatic geomorphologists (Budel, Peltier, Tricart and Cailleux) have divided the world into definite morphogenetic (climatogenetic) regions on the basis of dominant weathering and erosional processes generated by a particular suite of climatic parameters.

Question 121

Fundamental Concepts of Geomorphology: Concept 9: Climatic Geomorphology: major themes?

Answer 121

1. Landforms differ significantly in different climatic regions.
2. Spatial variations of landforms in differ ent climatic regions are because of spatial variations in climatic parameters (e.g. temperature, humidity, precipitation etc.) and their influences on weather ing, erosion and runoff.
3. Quaternary climatic changes could not obscure relationships between landforms and climates. In other words, there are certain diagnostic landforms which clearly demonstrate climate landforms relationships.

Question 122

Fundamental Concepts of Geomorphology: Concept 9: Climatic Geomorphology: Diagnostic landforms?

Answer 122

• diagnostic landforms are regarded as representatives of a particular climate.
• Climatogenetic or climatically controlled landforms are identified and differentiated in two ways e.g.
  
  • general observation and acquaintance of whole landscape of each climatic region, and
  • identification of typical or distinctive landforms which represent the control of a particular climate.
• The typical landforms are, in fact, main tools of climatic geomorphologists which help them in determining climate - landforms relationships in different climatic regions. Such distinctive landforms are designated as diagnostic landforms. The diag nostic landforms, identified by the climatic geomor phologists so far include inselbergs, duricrusts, ped iments, tors etc.
• Duricrusts:
  
  • Duricrusts are indurated hardened surfaces of different kinds such as laterites, silcretes, etc. depending on dominance of constituent minerals.
  • Normally, lateritic crusts are supposed to have been formed in hot and humid climate of tropical and subtropical areas and therefore these are indicative of hot and humid climates.
  • Lateritic crusts are predominantly found in Chotanagpur highlands (Patlands of Ranchi and Palamau plateaus) of Bihar (India) and over many areas of Decean plateau (e.g. Mahabaleshwar and Panchgani plateaus of Maharashtra).
  • The presence of lateritic crusts in certain parts of Europe (e.g. U.K., Germany etc.) clearly demonstrates the fact that these are not the result of the present climate.
  • ‘Such crusts are often interpreted as of Tertiary age, or as having been under continuous formation since the end of the Mesozoic.
• Inselbergs
  
  • They are steep sided residual hills and are considered to be the representative land forms of hot and arid and semi-arid climates and the end product of arid cycle of erosion but inselberg have been found in different parts of the world having different ‘climatic conditions, from humid subtropical in Georgia, North America to humid tropical in the Guinea coastlands, south India, Brazil, and to desert areas in western North America Mauretania, and south-west Africa’
  • It is argued that inselbergs are structurally controlled rather than climatically controlled and most of the present inselbergs were formed before Quaternary epoch, ‘hence present climates are not necessarily those in which the inselbergs were formed (D.R. Stoddart). It may be possible that inselbergs might have been formed when the climate was arid or semi-arid which might have changed after their formation.
• Pediments:
  
  • characterized by low-angle rock cut surfaces surrounding mountains, are considered to be the representative landforms of arid (desert) and semi-arid climates. Pediments are also found in a variety of climatic conditions e.g. tropical wet and dry climate, subtropical and temperate climate.
  • A few geomorphologists (e.g. W. Penck) argue that pediments are structurally and tectonically rather than climatically controlled. L.C. King has opined that the process of pediplanation and pedimentation is universal and it occurs in all environmental conditions.
• Tors:
  
  • ‘one of the most controvercial land forms, are piles of broken and exposed masses of hard rocks particularly granites having a crown of rock-blocks of different sizes on the tops and clitters (trains of blocks) on the sides’
  • Tors have been considered to be of periglacial origin by J. Palmer and R.A. Neilson (1962), of fluvial origin (humid climate, deep chemical weath ering and exhumation of rock debris by running water) by D.L. Linton (1955), whereas L.C. King has opined that tors are the result of universal proc ess of pediplanation in different climatic conditions.

Question 123

Fundamental Concepts of Geomorphology: Concept 9: Climatic Geomorphology: Direct control of climate?

Answer 123

• Temperature: Different morphogenetic processes:
  
  • eg. in regions with below Freezing point temperatures
    
    • If temperature (mean) of a region is below freezing point (less than 1°C), then there is frequent and wide spread frosting.
    • If there is such fluctuation in daily temperature that it goes down below freezing point during night but rises above freezing point during day time, then there occurs diurnal freeze (during night) and thaw (during day) cycle which leads to alternate processes of contration and expansion. Repitition of this causes frost weathering in Periglacial climate called Congelifraction; leads to tor formation where rocks are widely jointed.
    • Frost formation also impacts surface runoff and underground drainage (formation of stone streams) as well as aeolian process (niceo-aeolian deposits)
    • also decreases effectiveness of coastal processes by hardening coastal rocks
  • in regions with above freezing point temperatures
    
    • diurnal range of temperature facilitates weathering of rocks.
    • High diurnal range of tem perature leading to repetition of expansion and con traction for longer duration causes flaking in the rocks wherein thin sheets of rocks are peeled off layer after layer, the process is called exfoliation or onion weathering. This process is not only confined to hot desert areas but is also operative in monsoon climates. For example, the case of flaking and exfo liation weathering can well be seen over exposed granito-gneissic domes of Chotanagpur in general and Ranchi plateau in particular.
• Humidity and Precipitation
  
  • The amount, intensity and periodicity of rainfall are significant aspects which control and condition denudational processes in climates characterized by seasonality.
  • The area having clays gives birth to polygons when dehydrated due to high temperature during dry condition. Rainwater reaches the depth of 2-3 m through the cracks of such polygons and collects at the base where the geomaterial is more wet and relatively impermeable. Thus, the water at the base of poly gons becomes sliding plane and stimulates earth flow wherein polygons just above the sliding plane move downslope. Such geomorphic activities are operative in the areas of frequent alluviation during floods in the alluvial flood plains of rivers in mon soon climate (e.g. India). Mediterranean climatic regions, characterized by marked contrast in wet and dry seasons, present ideal conditions for such geo morphic mechanism i.e. slumping and earthflow.

Question 124

Fundamental Concepts of Geomorphology: Concept 9: Climatic Geomorphology: Indirect control of climate?

Answer 124

• Through Vegetation